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>

web/static/css/app.css

@@ -777,3 +777,11 @@ input[type="checkbox"] { accent-color: var(--accent); width: 18px; height: 18px;
 @media (prefers-reduced-motion: reduce) {
   *, *::before, *::after { animation-duration: 0.01ms !important; transition-duration: 0.01ms !important; }
 }
+
+/* ---------- settings: backups ---------- */
+.backup-status { font-size: 14px; }
+.backup-list { list-style: none; margin: 0; padding: 0; border-top: 1px solid var(--border); }
+.backup-list li { display: flex; align-items: center; gap: 12px; padding: 10px 0; border-bottom: 1px solid var(--border); }
+.backup-list .backup-when { flex: 1; min-width: 0; font-size: 14px; font-weight: 550; }
+.backup-list .backup-meta { display: block; color: var(--muted); font-size: 12.5px; font-weight: 400; margin-top: 2px; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+.backup-list .backup-actions { display: flex; gap: 2px; flex: none; }