fix(database): retire a table whose shape the schema moved past

`applySchema` is CREATE ... IF NOT EXISTS and there is no migration
chain, so a *changed* table never migrates: the statement silently
no-ops against the old shape. Two plans had already landed on that, and
neither showed up in a test because a fresh install is perfectly
healthy.

- 014 added `total_tracks` to explore_index and to `indexRowFields`,
  the projection every explore read uses, so every search, browse,
  artist page and album page failed with "no such column: total_tracks"
  on any database that already had a catalog.
- 013 reshaped audio_files, so applySchema could not run at all and the
  app did not open.

staleshape.go runs before applySchema and drops what disagrees, so the
create is a create. It parses sql/schemas/ for the expectation rather
than writing the column list down a second time, and it notices a
changed *type* as well as a missing column — 013 moved mbid TEXT to
BLOB, which no ALTER could express and which SQLite will not coerce, so
a query against 16 raw bytes returns no rows rather than an error.

Only Authored tables are exempt. Cache is rebuildable by definition,
Owned is what a rescan rebuilds (plan 013's stated "delete and
rescan"), and a table the schema no longer describes at all goes too --
013 left seven behind plus schema_migrations.

Three things in it are load-bearing, and each was a bug first:

- The parser read `UNIQUE(mbid)` as a column, which made a healthy
  catalog look stale. That would have retired it on every launch and
  cost every user an artifact download per start.
- The drops are one transaction with defer_foreign_keys. Those legacy
  tables reference each other, so any order fails on whichever goes
  first; turning foreign keys off instead would suppress
  playlist_tracks.audio_file_id's ON DELETE SET NULL and leave entries
  pointing at ids a rescan reissues to *different songs*. Nulled
  entries are empty; stale ones are wrong, and wrong quietly.
- The order is sorted, so a failure reproduces. Map order is random,
  and the foreign-key bug passed its own regression test on two runs in
  three until the order was fixed.

Verified against a real pre-013 install: it opens, its 22 playlists
survive, 1,887 linked playlist entries become 0 rather than dangling,
and the legacy tables are swept.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AfVYUVExXsx1nSWrXN8mAh

backend/database/database.go

@@ -89,6 +89,14 @@ func NewDB(logger *slog.Logger) (*DB, error) {
 		return nil, fmt.Errorf("could not apply PRAGMAs: %w", err)
 	}
 
+	// Before the schema is applied, not after: applySchema is
+	// CREATE ... IF NOT EXISTS, which no-ops against a table that
+	// already exists in an older shape.  Retiring the stale one first is
+	// what turns that no-op into a create.
+	if err := retireStaleTables(dbCtx, db, logger); err != nil {
+		return nil, err
+	}
+
 	if err := applySchema(dbCtx, db); err != nil {
 		return nil, err
 	}