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/engine/organize_test.go

@@ -20,23 +20,26 @@ func TestRenderName(t *testing.T) {
 	}
 	cases := []struct {
 		name, multi, ext string
+		digits           int
 		fields           map[string]string
 		want, missing    string
 	}{
-		{"basic", "prefix", ".mp3", f("artist", "Avenged Sevenfold", "album", "Nightmare", "track", "1", "title", "Nightmare"), "Avenged Sevenfold/Nightmare/01 Nightmare.mp3", ""},
-		{"album artist wins", "prefix", ".flac", f("artist", "Avenged Sevenfold feat. X", "album_artist", "Avenged Sevenfold", "album", "A", "track", "3", "title", "T"), "Avenged Sevenfold/A/03 T.flac", ""},
-		{"no track number", "prefix", ".mp3", f("artist", "A", "album", "B", "title", "Song"), "A/B/Song.mp3", ""},
-		{"missing album", "prefix", ".mp3", f("artist", "A", "title", "Song"), "", "album"},
-		{"missing title", "prefix", ".mp3", f("artist", "A", "album", "B", "track", "1"), "", "title"},
-		{"multi-disc prefix", "prefix", ".mp3", f("artist", "A", "album", "B", "track", "4", "disc", "2", "disc_total", "2", "title", "S"), "A/B/2-04 S.mp3", ""},
-		{"multi-disc folder", "folder", ".mp3", f("artist", "A", "album", "B", "track", "4", "disc", "2", "title", "S"), "A/B/Disc 2/04 S.mp3", ""},
-		{"single disc of one", "prefix", ".mp3", f("artist", "A", "album", "B", "track", "4", "disc", "1", "disc_total", "1", "title", "S"), "A/B/04 S.mp3", ""},
-		{"unsafe characters", "prefix", ".MP3", f("artist", "AC/DC", "album", "Who Made Who?", "track", "7", "title", "Title: Sub \"Live\"..."), "AC-DC/Who Made Who/07 Title - Sub 'Live'.mp3", ""},
-		{"no hidden names", "prefix", ".mp3", f("artist", ".hidden", "album", "B", "title", "S"), "hidden/B/S.mp3", ""},
-		{"big albums use 3 digits", "prefix", ".mp3", f("artist", "A", "album", "B", "track", "7", "track_total", "120", "title", "S"), "A/B/007 S.mp3", ""},
+		{"basic", "prefix", ".mp3", 0, f("artist", "Avenged Sevenfold", "album", "Nightmare", "track", "1", "title", "Nightmare"), "Avenged Sevenfold/Nightmare/01 Nightmare.mp3", ""},
+		{"album artist wins", "prefix", ".flac", 0, f("artist", "Avenged Sevenfold feat. X", "album_artist", "Avenged Sevenfold", "album", "A", "track", "3", "title", "T"), "Avenged Sevenfold/A/03 T.flac", ""},
+		{"no track number", "prefix", ".mp3", 0, f("artist", "A", "album", "B", "title", "Song"), "A/B/Song.mp3", ""},
+		{"missing album", "prefix", ".mp3", 0, f("artist", "A", "title", "Song"), "", "album"},
+		{"missing title", "prefix", ".mp3", 0, f("artist", "A", "album", "B", "track", "1"), "", "title"},
+		{"multi-disc prefix", "prefix", ".mp3", 0, f("artist", "A", "album", "B", "track", "4", "disc", "2", "disc_total", "2", "title", "S"), "A/B/02-04 S.mp3", ""},
+		{"multi-disc folder", "folder", ".mp3", 0, f("artist", "A", "album", "B", "track", "4", "disc", "2", "title", "S"), "A/B/Disc 02/04 S.mp3", ""},
+		{"single disc of one", "prefix", ".mp3", 0, f("artist", "A", "album", "B", "track", "4", "disc", "1", "disc_total", "1", "title", "S"), "A/B/04 S.mp3", ""},
+		{"unsafe characters", "prefix", ".MP3", 0, f("artist", "AC/DC", "album", "Who Made Who?", "track", "7", "title", "Title: Sub \"Live\"..."), "AC-DC/Who Made Who/07 Title - Sub 'Live'.mp3", ""},
+		{"no hidden names", "prefix", ".mp3", 0, f("artist", ".hidden", "album", "B", "title", "S"), "hidden/B/S.mp3", ""},
+		{"unpadded disc prefix", "prefix", ".mp3", 1, f("artist", "A", "album", "B", "track", "4", "disc", "2", "disc_total", "2", "title", "S"), "A/B/2-04 S.mp3", ""},
+		{"unpadded disc folder", "folder", ".mp3", 1, f("artist", "A", "album", "B", "track", "4", "disc", "2", "title", "S"), "A/B/Disc 2/04 S.mp3", ""},
+		{"big albums use 3 digits", "prefix", ".mp3", 0, f("artist", "A", "album", "B", "track", "7", "track_total", "120", "title", "S"), "A/B/007 S.mp3", ""},
 	}
 	for _, c := range cases {
-		got, missing := renderName(DefaultOrganizeTemplate, c.multi, c.ext, c.fields)
+		got, missing := renderName(DefaultOrganizeTemplate, c.multi, c.digits, c.ext, c.fields)
 		if got != c.want || missing != c.missing {
 			t.Errorf("%s: got %q (missing %q), want %q (missing %q)", c.name, got, missing, c.want, c.missing)
 		}