feat: add Phase 2 observability and audit retention

[pppolf]

Summary

• add Alembic 0005 Worker progress facts, 13-table importer integrity gates, and DB-time Worker liveness metrics
• add bounded low-cardinality Prometheus exposition, eight alert rules, and fail-closed audit/history validation
• add canonical audit archive verify/reconcile/restore/delete CLI and closed structured logging contracts

Validation

• make lint
• make test: backend 916 passed, 33 skipped; frontend 38 passed
• make smoke: 1 passed, 7 deselected
• isolated PostgreSQL 16 and Redis 7 integration: 33 passed, 0 skipped
• clean-SHA Phase 2 acceptance: 9/9, evidence SHA-256 e4ffb8668fd3fa62d59b5d83f5c29eede35b327d88e6099345acd5950670fc47
• clean-SHA Phase 2 capacity: passed, evidence SHA-256 2382f9138f09028f269d76c341b236dd4089d678c8a2323582045fac2b4f5039

All automated model behavior is Mock-only. P2-06 and Phase 2 remain in progress until exact-SHA CI and evidence closeout complete; P2-07 backup/restore remains out of scope.