fix(db): backfill empty episode titles to TBA

crates/arr-db/migrations/0021_episode_title_required.sql

@@ -0,0 +1,11 @@
+-- #153. `episodes.title` accepted the empty string, which TMDB sends for an
+-- unaired episode it has not named yet. Empty titles leaked into §9.2's
+-- search haystack, §7.4 filenames and the compat shim as if they were real
+-- text. Rows already carrying `''` take the same "TBA" placeholder the TMDB
+-- boundary now substitutes — #121's guarded update replaces it once TMDB
+-- fills the title in.
+--
+-- A hard CHECK (title <> '') would need a table rebuild with foreign_keys
+-- off, which sqlx 0.8's migrator cannot run (it always wraps a migration in
+-- a transaction, where that pragma is a no-op); enforcement lives in code.
+UPDATE episodes SET title = 'TBA', updated_at = strftime('%Y-%m-%dT%H:%M:%fZ', 'now') WHERE title = '';