Add HTTP REST API gateway for Synor Database L2:
- Gateway server with Axum HTTP framework
- API key authentication with permissions and rate limiting
- Full REST endpoints for all database models:
- Key-Value: GET/PUT/DELETE /kv/:key, POST /kv/batch
- Documents: CRUD operations, MongoDB-style queries
- Vectors: embedding insert, similarity search
- Time-series: metrics recording and queries
- Usage metering for billing integration
- CORS and request timeout configuration
All 51 tests passing. Phase 10 now complete (100%).
