feat: financial data retention & aggregation system
Add CleanupExpiredFinancialRecords to enforce HMRC + Limitation Act
compliance (7-year retention, 1-year post-anonymization buffer).
- financial_aggregates table: monthly totals by payment method/type (no PII)
- CleanupExpiredFinancialRecords(): aggregates expired payments/refunds,
deletes granular records, idempotent via ON CONFLICT DO UPDATE
- Wired into GET /api/availability alongside existing cleanup functions
- 8 tests: 7yr expiry, 1yr buffer, 9yr override, aggregation totals,
idempotency, active user protection, both-thresholds elapsed, refunds
- testdb.go: financial_aggregates in drop-order and truncate lists
- README + Technical Manual updated
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