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>

server/src/index.js

@@ -9,6 +9,7 @@ const paychecksRouter = require('./routes/paychecks');
 const actualsRouter = require('./routes/actuals');
 const oneTimeExpensesRouter = require('./routes/one-time-expenses');
 const summaryRouter = require('./routes/summary');
+const { router: financingRouter } = require('./routes/financing');
 const db = require('./db');
 
 const app = express();
@@ -25,6 +26,7 @@ app.use('/api', paychecksRouter);
 app.use('/api', actualsRouter);
 app.use('/api', oneTimeExpensesRouter);
 app.use('/api', summaryRouter);
+app.use('/api', financingRouter);
 
 // Serve static client files in production
 const clientDist = path.join(__dirname, '../../client/dist');