feat: persist audit logs and expose escrow event projection (#198, #206)

[temitope-007]

This PR resolves two related backend infrastructure tasks to improve durability and caching:

Audit Log Persistence

• Persists audit logs to the durable audit_log_events table instead of the in-memory array in src/services/auditLog.js.
• Respects the append-only table configuration and check constraints.
• Updates the invoice state machine and routes to handle asynchronous logging securely.

Escrow Event Projection

• Exposes the escrow event projection through GET /api/escrow/:invoiceId.
• Reads from escrow_event_projection keyed by invoice_id, including latest_ledger_sequence and latest_event_type.
• Honors the existing Redis escrow cache and falls back to a live read only on cache and projection misses.
• Maintains the X-Escrow-Address header.

Both feature sets achieve minimum 95% line coverage on changed code and pass all tests. No secrets have been exposed.

Closes #198
Closes #206

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