Add special financing feature

Data model:
- Migration 003: financing_plans and financing_payments tables
- Plans track total amount, due date, paycheck assignment (1, 2, or split)
- Payments linked to paycheck records, one per period

Payment calculation:
- Auto-calculated: (total - paid_so_far) / remaining_periods
- Split plans halve the per-period amount across both paychecks
- Recalculates each period as payments are made

Financing page (/financing):
- Create/edit/delete plans with name, amount, due date, paycheck assignment
- Progress bar showing paid vs total
- Overdue badge when past due with remaining balance
- Paid-off plans moved to a separate section

Paycheck view:
- New Financing section per column with payment checkbox
- Overdue badge on individual payments
- Running paid/total shown in bill meta
- Financing payments included in remaining balance calculation
- Auto-closes plan and reloads when fully paid off
- Lazy generation works: first interaction generates paycheck and payment records

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

db/migrations/003_financing.sql

@@ -0,0 +1,21 @@
+-- financing_plans: tracks a deferred/no-interest financing arrangement
+CREATE TABLE IF NOT EXISTS financing_plans (
+  id                SERIAL PRIMARY KEY,
+  name              TEXT NOT NULL,
+  total_amount      NUMERIC(12, 2) NOT NULL,
+  due_date          DATE NOT NULL,          -- must be paid off by this date
+  assigned_paycheck INTEGER,               -- 1, 2, or NULL (split across both)
+  active            BOOLEAN NOT NULL DEFAULT TRUE,
+  created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- financing_payments: one row per paycheck period for each active plan
+CREATE TABLE IF NOT EXISTS financing_payments (
+  id          SERIAL PRIMARY KEY,
+  plan_id     INTEGER NOT NULL REFERENCES financing_plans(id) ON DELETE CASCADE,
+  paycheck_id INTEGER NOT NULL REFERENCES paychecks(id) ON DELETE CASCADE,
+  amount      NUMERIC(12, 2) NOT NULL,     -- calculated at generation time
+  paid        BOOLEAN NOT NULL DEFAULT FALSE,
+  paid_at     TIMESTAMPTZ,
+  UNIQUE (plan_id, paycheck_id)
+);