feat(02-05): Payment APIs, outstanding report, and 29 passing tests
- Implement OutstandingReportService.getOutstandingReport() with filtering/pagination
- Add POST/GET /api/payments (record and list payments)
- Add GET /api/payments/[id] (payment detail)
- Add POST /api/payments/[id]/void (void payment)
- Add GET /api/subscribers/[id]/payments (payment history)
- Add GET /api/subscribers/[id]/balance (outstanding balance)
- Add GET /api/reports/outstanding (outstanding report)
- Write 29 comprehensive tests covering full/partial/overpayment,
FIFO allocation, JE balance, bank vs cash, idempotency, void,
outstanding report filters, reconciliation, history, tenant isolation
- All 265 tests pass (full regression clean)
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