Add scheduled and manual backups

Backups are .tar.gz archives of a consistent database snapshot (VACUUM
INTO) plus the images folder. They run on a cron schedule, daily at 3:00
by default, and the newest N scheduled ones are kept (7 by default).
Manual backups are never pruned.

Settings → Backups offers schedule presets or a custom cron expression,
retention, "Back up now", and download/delete for each backup. A small
built-in parser handles standard 5-field cron syntax with names, ranges,
steps and @daily-style shorthands.

Backups go to CANON_BACKUP_DIR (default data/backups). Compose mounts a
separate canon-backups volume at /backups, overridable with
CANON_BACKUP_PATH.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -150,10 +150,21 @@ Canon was called TopDeck, and upgrading keeps all your data:
 
 ## Backups
 
-Everything lives in the data volume: `canon.db` (SQLite) and `images/`. Copy the volume, or while running:
+Canon backs itself up every day at 3:00 and keeps the last 7. Change this under **Settings → Backups**: pick a preset (hourly, every 6 hours, daily, weekly, monthly), write your own cron expression, or turn scheduled backups off. **Back up now** makes a manual backup; manual backups are never deleted automatically. Every backup can be downloaded or deleted from the same page.
+
+Each backup is a `.tar.gz` holding `canon.db` (a consistent snapshot, taken while Canon keeps running) and the `images/` folder.
+
+- **Where:** `CANON_BACKUP_DIR`, by default `data/backups`. Docker Compose mounts the `canon_canon-backups` volume at `/backups`; set `CANON_BACKUP_PATH` to a host folder (a NAS share, say) to keep them off the data volume.
+- **Time zone:** schedules use the server's time zone, UTC in Docker unless you set `TZ` (e.g. `TZ=America/Los_Angeles`) in `.env`.
+- **Cron syntax:** `minute hour day month weekday`, with `*`, lists (`1,15`), ranges (`mon-fri`), steps (`*/6`) and `@daily`-style shorthands. For example, `30 2 * * mon-fri` is 2:30 on weekdays.
+
+To restore, stop Canon, extract a backup into the data folder, and start it again:
 
 ```sh
-docker compose exec canon sh -c 'cd /data && tar c canon.db* images' > canon-backup.tar
+docker compose stop canon
+docker run --rm -v canon_canon-data:/data -v canon_canon-backups:/backups alpine \
+  sh -c 'rm -f /data/canon.db-wal /data/canon.db-shm && tar xzf /backups/<file>.tar.gz -C /data && chown -R 1000:1000 /data'
+docker compose start canon
 ```
 
 ## Development