.git_commit_msg_issue333

feat(#333): Implement revocation checkpoint commitments with monotonic enforcement

- Add RevocationCheckpoint and RevocationCheckpointStreamHead Prisma models
- Implement database-level monotonicity enforcement via unique constraints
- Persist quarantined checkpoints (regressed roots, conflicts) with bounded reason codes
- Provide freshness DTO with age calculation and staleness detection
- API endpoints: POST /revocation-checkpoints, GET /fresh, GET /quarantine
- RevocationCheckpointService with monotonicity logic and quarantine handling
- Comprehensive test suite: monotonicity, concurrency, validation, quarantine lifecycle
- DTOs with field validation (ledger, algorithm, root format, observedAt)
- Integration with Proofs module; proof verification isolation maintained
- Migration 20261215100000 with tables and indexes
- Complete documentation in docs/revocation-checkpoints.md

Acceptance Criteria Satisfied:
- Checkpoint persisted with sequence, root, algorithm, ledger, contract identity
- Monotonicity enforced at DB level (unique constraint + conditional logic)
- Regressed roots quarantined, never silently accepted or dropped
- Freshness and live reconciliation exposed via DTO
- Proof verification unchanged (checkpoint is additional evidence only)
- Concurrent writers safe (one accepted, rest idempotent or quarantined)
- Deterministic error semantics with bounded codes
- Synthetic test fixtures only (no secrets, no production data)