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>

.gitignore

@@ -2,6 +2,7 @@ node_modules/
 .svelte-kit/
 build/
 data/
+backups/
 *.log
 .env
 .env.*