Security hardening, backup restore fix & restore warning modal

- Fix backup restore: try/finally ensures DB always reopens after closeDb
- Fix EBUSY on uploads during restore (in-place overwrite instead of rmSync)
- Add DB proxy null guard for clearer errors during restore window
- Add red warning modal before backup restore (DE/EN, dark mode support)
- JWT secret: empty docker-compose default so auto-generation kicks in
- OIDC: pass token via URL fragment instead of query param (no server logs)
- Block SVG uploads on photos, files and covers (stored XSS prevention)
- Add helmet for security headers (HSTS, X-Frame, nosniff, etc.)
- Explicit express.json body size limit (100kb)
- Fix XSS in Leaflet map markers (escape image_url in HTML)
- Remove verbose WebSocket debug logging from client

server/package.json

@@ -12,6 +12,7 @@
     "cors": "^2.8.5",
     "dotenv": "^16.4.1",
     "express": "^4.18.3",
+    "helmet": "^8.1.0",
     "jsonwebtoken": "^9.0.2",
     "multer": "^1.4.5-lts.1",
     "node-cron": "^4.2.1",