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/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "lisilou-api",
+  "version": "1.0.0",
+  "main": "server.js",
+  "scripts": {
+    "start": "node server.js",
+    "dev": "node --watch server.js"
+  },
+  "dependencies": {
+    "better-sqlite3": "^9.4.3",
+    "cors": "^2.8.5",
+    "dotenv": "^16.4.5",
+    "express": "^4.18.3"
+  }
+}