Add database backups and two-digit disc numbers

Disc numbers: the organizer writes two-digit disc numbers by default
(Disc 01, 01-04 Title, {disc}), with an option for the plain style
(Disc 2, 2-04). Migration 8 adds the per-pair setting.

Backups (Settings -> Backups), similar to Sonarr:
- A backup is a zip with a consistent snapshot of the database (VACUUM
  INTO, safe while running) and backup.json. Configurable folder,
  schedule (off/1/3/7/14 days) and retention for scheduled backups;
  manual backups are kept until deleted. List, download, delete.
- Restore from a listed backup or an uploaded zip: validated first
  (integrity check, playlink tables, not a newer schema), staged next to
  the live database, then the server restarts; at startup the staged
  database is swapped in and the previous one kept as
  playlink.db.pre-restore-<time>. Refused while a job runs. Backup names
  are validated, so downloads/deletes cannot reach other files.
- Optional backup before every action in live mode: once per action the
  user starts (link, promote, bulk, build/remove, tag edit, folder
  renames, undo, purge) and before scans auto-link or auto-organize,
  never per file and never in dry run. If the backup fails the action
  does not run. The newest 10 action backups are kept.

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

internal/store/store.go

@@ -126,8 +126,14 @@ var migrations = []string{
 	`ALTER TABLE pairs ADD COLUMN organize_template TEXT NOT NULL DEFAULT '';
 	ALTER TABLE pairs ADD COLUMN organize_multidisc TEXT NOT NULL DEFAULT 'prefix';
 	ALTER TABLE pairs ADD COLUMN auto_organize TEXT NOT NULL DEFAULT 'off';`,
+
+	// 8: zero-padded disc numbers (Disc 01) by default.
+	`ALTER TABLE pairs ADD COLUMN organize_disc_digits INTEGER NOT NULL DEFAULT 2;`,
 }
 
+// SchemaVersion is the newest database schema this build knows.
+func SchemaVersion() int { return len(migrations) }
+
 // Open opens (and migrates) the database in dataDir.
 func Open(dataDir string) (*Store, error) {
 	dsn := "file:" + filepath.Join(dataDir, "playlink.db") +