Add automatic backups and an admin restore page

Scheduler ticks every 5 minutes and snapshots data/avtoambor.db (via
better-sqlite3's online backup API) when the DB file's mtime has advanced.
After each new backup, prune older snapshots: keep everything from the last
7 days, then one per calendar day. New /admin page lists backups with
Download and Restore actions, plus a Back-up-now button. Restore takes a
safety snapshot first, closes the live connection, swaps the .db file, and
lets the next request reopen.

Also: TZ=Asia/Dushanbe in the container so backup filenames use local time,
and tzdata added to the image so TZ takes effect.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -15,4 +15,5 @@ services:
       - ./data:/app/data
     environment:
       - NODE_ENV=development
+      - TZ=Asia/Dushanbe
     command: npm run dev