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>

nginx.conf

@@ -86,6 +86,18 @@ http {
             proxy_ssl_server_name on;
         }
 
+        # Booking API — proxy to Node.js service
+        # Immich regex locations above take priority for /api/immich-proxy and /api/fetch-immich/
+        location /api/ {
+            proxy_pass http://api:3001;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+            proxy_read_timeout 30s;
+        }
+
         # Proxy endpoint for Immich share pages and API
         location ~ ^/api/fetch-immich/(.+)$ {
             resolver 8.8.8.8 1.1.1.1 valid=300s;