fix: multiple production issues with PostgreSQL and connection handling

- Fix trust proxy to use specific values instead of permissive 'true'
- Fix clear old notifications to use database-agnostic date calculation
- Fix database size check to support both PostgreSQL and SQLite
- Add caching and better error handling for session timeout queries
- Add query timeout to prevent hanging connections
- Improve JSON parsing error handling for setting values

These fixes address:
- ERR_ERL_PERMISSIVE_TRUST_PROXY warning
- PostgreSQL datetime function errors
- ENOENT errors looking for SQLite file in PostgreSQL deployment
- Connection terminated errors for session timeout checks

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/server.js

@@ -29,7 +29,8 @@ const app = express();
 const PORT = process.env.PORT || 3000;
 
 // Trust proxy headers (required for Traefik/nginx)
-app.set('trust proxy', true);
+// Set to specific number of proxies or loopback to be more secure
+app.set('trust proxy', 'loopback, linklocal, uniquelocal');
 
 // Security middleware with custom CSP
 app.use(helmet({