Merge pull request 'Docs: backfill JSDoc, utility docs, and CLAUDE.md API/schema sections' (#6) from docs/backfill-missing-documentation into master

Reviewed-on: #6

CLAUDE.md

@@ -94,3 +94,87 @@ The default route `/` renders the paycheck-centric main view (`client/src/pages/
 **Financing:** `GET/POST /api/financing`, `PUT/DELETE /api/financing/:id`, `PATCH /api/financing-payments/:id/paid`. Plans track a total amount, payoff due date, and `start_date`. Payment per period is auto-calculated as `(remaining balance) / (remaining periods)`. Split plans (`assigned_paycheck = null`) divide each period's payment across both paychecks. Plans auto-close when fully paid. Financing payments are included in the paycheck remaining balance. `start_date` prevents a plan from appearing on paycheck months before it was created — both virtual previews and `generate` respect this guard.
 
 **Migrations:** SQL files in `db/migrations/` are applied in filename order on server startup. Add new migrations as `00N_description.sql` — they run once and are tracked in the `migrations` table.
+
+## Database Schema
+
+Full DDL lives in `db/migrations/`. Key tables:
+
+| Table | Description |
+|---|---|
+| `config` | Key/value store for app settings (paycheck days, gross/net amounts). |
+| `bills` | Bill definitions: name, amount, due day, assigned paycheck, category, active flag. |
+| `paychecks` | One row per paycheck per period (year + month + paycheck_number 1 or 2). |
+| `paycheck_bills` | Junction between a paycheck instance and its bills; tracks paid status and amount_override. |
+| `one_time_expenses` | Ad-hoc expenses attached to a specific paycheck instance. |
+| `financing_plans` | Financing plans: total amount, due date, start_date, optional assigned_paycheck. |
+| `financing_payments` | One payment record per plan per paycheck; tracks paid status. |
+| `expense_categories` | Lookup table for variable-expense categories (Groceries, Gas, Dining, …). |
+| `actuals` | Actual spending log entries linked to a paycheck, category, or bill. |
+| `migrations` | Internal table tracking which migration files have been applied. |
+
+## API Endpoints
+
+All routes are prefixed with `/api`.
+
+### Paychecks
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/paychecks?year=&month=` | Return both paychecks for a month. Returns virtual data (id: null) when no DB record exists. |
+| `POST` | `/paychecks/generate?year=&month=` | Upsert paycheck records and sync bills/financing for the month. |
+| `GET` | `/paychecks/months` | List all months that have generated paycheck records, newest first. |
+| `PATCH` | `/paychecks/:id` | Update gross and net for a specific paycheck. |
+| `PATCH` | `/paycheck-bills/:id/paid` | Toggle paid status; locks amount_override on pay. |
+| `PATCH` | `/paycheck-bills/:id/amount` | Set amount_override for a variable-amount bill. |
+
+### Bills
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/bills` | List all bills ordered by assigned_paycheck, name. |
+| `POST` | `/bills` | Create a bill. |
+| `GET` | `/bills/:id` | Fetch a single bill. |
+| `PUT` | `/bills/:id` | Replace a bill's fields. |
+| `DELETE` | `/bills/:id` | Hard-delete a bill. |
+| `PATCH` | `/bills/:id/toggle` | Toggle the active flag. |
+
+### Config
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/config` | Return all config values as a flat object with numeric values. |
+| `PUT` | `/config` | Upsert one or more config keys. Unknown keys are silently ignored. |
+
+### One-Time Expenses
+| Method | Path | Description |
+|---|---|---|
+| `POST` | `/one-time-expenses` | Add a one-time expense to a paycheck. |
+| `DELETE` | `/one-time-expenses/:id` | Remove a one-time expense. |
+| `PATCH` | `/one-time-expenses/:id/paid` | Toggle paid status. |
+
+### Financing
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/financing` | List all financing plans with enriched progress fields. |
+| `POST` | `/financing` | Create a financing plan. |
+| `GET` | `/financing/:id` | Fetch a plan with its full payment history. |
+| `PUT` | `/financing/:id` | Update a financing plan. |
+| `DELETE` | `/financing/:id` | Delete a financing plan and its payments. |
+| `PATCH` | `/financing-payments/:id/paid` | Toggle a payment's paid status; auto-closes the plan when fully paid. |
+
+### Actuals & Categories
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/expense-categories` | List all expense categories. |
+| `POST` | `/expense-categories` | Create a new expense category. |
+| `GET` | `/actuals?paycheckId=` | List actual spending entries for a paycheck. |
+| `POST` | `/actuals` | Log an actual spending entry. |
+| `DELETE` | `/actuals/:id` | Remove an actual spending entry. |
+
+### Summary
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/summary/monthly?year=&month=` | Spending breakdown and category totals for a single month. |
+| `GET` | `/summary/annual?year=` | Income vs. spending, surplus/deficit trend, and stacked variable spending for a full year. |
+
+### Health
+| Method | Path | Description |
+|---|---|---|
+| `GET` | `/health` | Returns `{ ok: true }`. Used by Docker healthcheck. |

client/src/ThemeContext.jsx

@@ -2,6 +2,18 @@ import { createContext, useContext, useEffect, useState } from 'react';
 
 const ThemeContext = createContext(null);
 
+/**
+ * Provides light/dark theme state to the component tree.
+ *
+ * On mount, the active theme is read from `localStorage`; if absent it
+ * falls back to the OS `prefers-color-scheme` media query. The chosen
+ * theme is applied as a `data-theme` attribute on `<html>` and persisted
+ * to `localStorage` whenever it changes.
+ *
+ * Exposes `{ theme, toggle }` via {@link useTheme}.
+ *
+ * @param {{ children: React.ReactNode }} props
+ */
 export function ThemeProvider({ children }) {
   const [theme, setTheme] = useState(() => {
     const stored = localStorage.getItem('theme');
@@ -25,6 +37,13 @@ export function ThemeProvider({ children }) {
   );
 }
 
+/**
+ * Returns the current theme context provided by {@link ThemeProvider}.
+ *
+ * @returns {{ theme: 'light'|'dark', toggle: () => void }}
+ *   - `theme`  — the active color scheme name
+ *   - `toggle` — flips between `'light'` and `'dark'`
+ */
 export function useTheme() {
   return useContext(ThemeContext);
 }

server/src/routes/bills.js

@@ -2,6 +2,20 @@ const express = require('express');
 const router = express.Router();
 const { pool } = require('../db');
 
+/**
+ * Validate the request body for bill create/update operations.
+ *
+ * Checks that all required fields are present and within acceptable ranges.
+ * Amount is optional when `variable_amount` is true (defaults to 0 on save).
+ *
+ * @param {object} body - Request body.
+ * @param {string}        body.name              - Bill name (non-empty).
+ * @param {number|string} [body.amount]           - Bill amount; required when variable_amount is falsy.
+ * @param {number|string} body.due_day            - Day of month (1–31).
+ * @param {number|string} body.assigned_paycheck  - Which paycheck: 1 or 2.
+ * @param {boolean}       [body.variable_amount]  - Whether the bill amount varies each month.
+ * @returns {string|null} Validation error message, or null when valid.
+ */
 function validateBillFields(body) {
   const { name, amount, due_day, assigned_paycheck, variable_amount } = body;
   if (!name || name.toString().trim() === '') {

server/src/routes/config.js

@@ -16,6 +16,22 @@ const DEFAULTS = {
   paycheck2_day: 15,
 };
 
+/**
+ * Fetch all application config values from the database.
+ *
+ * Reads all known `CONFIG_KEYS` from the `config` table and coerces values
+ * to numbers. Keys missing from the DB fall back to hard-coded `DEFAULTS`
+ * (paycheck1_day = 1, paycheck2_day = 15); keys with no default are null.
+ *
+ * @returns {Promise<{
+ *   paycheck1_day: number|null,
+ *   paycheck2_day: number|null,
+ *   paycheck1_gross: number|null,
+ *   paycheck1_net: number|null,
+ *   paycheck2_gross: number|null,
+ *   paycheck2_net: number|null
+ * }>}
+ */
 async function getAllConfig() {
   const result = await pool.query(
     'SELECT key, value FROM config WHERE key = ANY($1)',

server/src/routes/financing.js

@@ -4,9 +4,20 @@ const { pool } = require('../db');
 
 // ─── Helpers ──────────────────────────────────────────────────────────────────
 
-// Count how many payment periods remain for a plan starting from (year, month),
+/**
-// including that month. Each month contributes 1 or 2 periods depending on
+ * Count the number of payment periods remaining for a plan, starting from
-// whether the plan is split across both paychecks (assigned_paycheck = null).
+ * (and including) the given year/month.
+ *
+ * Each calendar month contributes 1 period for single-paycheck plans
+ * (`assigned_paycheck` = 1 or 2) or 2 periods for split plans
+ * (`assigned_paycheck` = null). Always returns at least 1 to prevent
+ * division-by-zero in {@link calcPaymentAmount}.
+ *
+ * @param {{ due_date: string, assigned_paycheck: number|null }} plan
+ * @param {number} year  - Current year.
+ * @param {number} month - Current month (1–12).
+ * @returns {number} Number of remaining payment periods (≥ 1).
+ */
 function remainingPeriods(plan, year, month) {
   const due = new Date(plan.due_date);
   const dueYear  = due.getFullYear();
@@ -19,7 +30,18 @@ function remainingPeriods(plan, year, month) {
   return monthsLeft * perMonth;
 }
 
-// Calculate the payment amount for one period.
+/**
+ * Calculate the payment amount due for a single period.
+ *
+ * Formula: `(total_amount - paid_so_far) / remainingPeriods(plan, year, month)`.
+ * Returns 0 when the plan is already fully paid.
+ *
+ * @param {import('pg').PoolClient} client - Active DB client (read-only query).
+ * @param {{ id: number, total_amount: number|string, due_date: string, assigned_paycheck: number|null }} plan
+ * @param {number} year  - Current year for period calculation.
+ * @param {number} month - Current month (1–12) for period calculation.
+ * @returns {Promise<number>} Payment amount rounded to 2 decimal places, or 0.
+ */
 async function calcPaymentAmount(client, plan, year, month) {
   const { rows } = await client.query(
     `SELECT COALESCE(SUM(fp.amount), 0) AS paid_total
@@ -35,7 +57,22 @@ async function calcPaymentAmount(client, plan, year, month) {
   return parseFloat((remaining / periods).toFixed(2));
 }
 
-// Enrich a plan row with computed progress fields.
+/**
+ * Enrich a raw `financing_plans` row with computed progress fields.
+ *
+ * Aggregates payment records to derive `paid_total`, `remaining`,
+ * `paid_count`, `total_count`, and `overdue`. Used by every route that
+ * returns a plan to the client.
+ *
+ * @param {import('pg').Pool} pool - DB pool (runs a single SELECT).
+ * @param {object} plan - Raw row from the `financing_plans` table.
+ * @returns {Promise<object>} Plan object extended with:
+ *   - `paid_total`  {number} — sum of paid payment amounts
+ *   - `remaining`   {number} — total_amount minus paid_total (≥ 0)
+ *   - `paid_count`  {number} — number of paid financing_payments rows
+ *   - `total_count` {number} — total financing_payments rows for the plan
+ *   - `overdue`     {boolean} — true when active, remaining > 0, and due_date is in the past
+ */
 async function enrichPlan(pool, plan) {
   const { rows } = await pool.query(
     `SELECT

server/src/routes/paychecks.js

@@ -43,9 +43,29 @@ function pad2(n) {
   return String(n).padStart(2, '0');
 }
 
-// Build virtual (unsaved) paycheck data from config + active bills.
+/**
-// Returns the same shape as fetchPaychecksForMonth but with id: null
+ * Build virtual (unsaved) paycheck data from config + active bills.
-// and paycheck_bill_id: null — nothing is written to the DB.
+ *
+ * Returns the same shape as {@link fetchPaychecksForMonth} but with
+ * `id: null` and `paycheck_bill_id: null` — nothing is written to the DB.
+ * Financing payment previews are included with `financing_payment_id: null`.
+ * Plans whose `start_date` is after the requested period are excluded.
+ *
+ * @param {number} year  - Full four-digit year (e.g. 2025).
+ * @param {number} month - Month number 1–12.
+ * @returns {Promise<Array<{
+ *   id: null,
+ *   period_year: number,
+ *   period_month: number,
+ *   paycheck_number: 1|2,
+ *   pay_date: string,
+ *   gross: number,
+ *   net: number,
+ *   bills: Array<object>,
+ *   one_time_expenses: [],
+ *   financing: Array<object>
+ * }>>}
+ */
 async function buildVirtualPaychecks(year, month) {
   const config = await getConfig();
   const paychecks = [];
@@ -137,8 +157,19 @@ async function buildVirtualPaychecks(year, month) {
   return paychecks;
 }
 
-// Generate (upsert) paycheck records for the given year/month.
+/**
-// Returns the two paycheck IDs.
+ * Generate (upsert) paycheck records for the given year/month.
+ *
+ * Inserts or updates both `paychecks` rows, syncs `paycheck_bills` for all
+ * active bills, and inserts `financing_payments` for active plans that have
+ * started by this period. All writes run inside a single transaction.
+ * Split financing plans (assigned_paycheck = null) get half the per-period
+ * amount on each paycheck.
+ *
+ * @param {number} year  - Full four-digit year.
+ * @param {number} month - Month number 1–12.
+ * @returns {Promise<number[]>} Two-element array of paycheck IDs `[id1, id2]`.
+ */
 async function generatePaychecks(year, month) {
   const config = await getConfig();
 
@@ -221,7 +252,25 @@ async function generatePaychecks(year, month) {
   }
 }
 
-// Fetch both paycheck records for a month with full bill and one_time_expense data.
+/**
+ * Fetch both persisted paycheck records for a month with full bill,
+ * one-time-expense, and financing-payment data joined in.
+ *
+ * @param {number} year  - Full four-digit year.
+ * @param {number} month - Month number 1–12.
+ * @returns {Promise<Array<{
+ *   id: number,
+ *   period_year: number,
+ *   period_month: number,
+ *   paycheck_number: 1|2,
+ *   pay_date: string,
+ *   gross: number,
+ *   net: number,
+ *   bills: Array<object>,
+ *   one_time_expenses: Array<object>,
+ *   financing: Array<object>
+ * }>>} Empty array when no DB records exist for the given month.
+ */
 async function fetchPaychecksForMonth(year, month) {
   const pcResult = await pool.query(
     `SELECT id, period_year, period_month, paycheck_number, pay_date, gross, net