Add Node.js/Express API service for booking backend

- New api/ service: Express + better-sqlite3, health check at GET /api/health,
  POST /api/bookings stores sessions with all required fields
- Dockerfile with Alpine build deps for native sqlite3 module
- docker-compose.yml: api service with volume mounts for data, signed-contracts, contracts
- nginx.conf: proxy /api/ to api:3001; Immich regex locations remain higher priority
- .gitignore: exclude SQLite db file and signed PDFs from version control

Closes #1

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -29,3 +29,8 @@ config/local.json
 
 # Docker
 docker-compose.override.yml
+
+# API runtime data
+api/data/*.db
+api/signed-contracts/*.pdf
+api/.env