feat: add granular auto-backup scheduling and timezone support

Add UI controls for configuring auto-backup schedule with hour, day of
week, and day of month pickers. The hour picker respects the user's
12h/24h time format preference from settings.

Add TZ environment variable support via docker-compose so the container
runs in the configured timezone. The timezone is passed to node-cron for
accurate scheduling and exposed via the API so the UI displays it.

Fix SQLite UTC timestamp handling by appending Z suffix to all timestamps
sent to the client, ensuring proper timezone conversion in the browser.

Made-with: Cursor

docker-compose.yml

@@ -9,6 +9,7 @@ services:
       - JWT_SECRET=${JWT_SECRET:-}
       # - ALLOWED_ORIGINS=https://yourdomain.com  # Optional: restrict CORS to specific origins
       - PORT=3000
+      - TZ=${TZ:-UTC}
     volumes:
       - ./data:/app/data
       - ./uploads:/app/uploads