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>

api/.env.example

@@ -0,0 +1,21 @@
+PORT=3001
+CORS_ORIGIN=http://localhost:8080
+
+# n8n webhook for email notifications
+N8N_WEBHOOK_URL=
+
+# Google Calendar (issue #3)
+GOOGLE_CALENDAR_ID=
+GOOGLE_SERVICE_ACCOUNT_JSON=
+
+# Contract PDF signature placement (issue #6)
+CONTRACT_SIG_X=100
+CONTRACT_SIG_Y=700
+CONTRACT_SIG_PAGE=1
+
+# OIDC / Authentik (issue #10)
+SESSION_SECRET=
+OIDC_ISSUER=
+OIDC_CLIENT_ID=
+OIDC_CLIENT_SECRET=
+OIDC_REDIRECT_URI=