Add TVmaze as a second, user-selectable TV metadata source for Fetch Data

TV episodes can now resolve against either TMDB (default) or TVmaze - picked
via a segmented source picker in the renamed "Fetch Data Settings" modal.
TVmaze is free and needs no API token at all; movies still always resolve
against TMDB regardless of the choice, since TVmaze has no movie catalog.

- New TvmazeClient (crates/aurora-curator-core/src/web/tvmaze.rs): search +
  season/episode lookup against api.tvmaze.com, filtering out unnumbered
  specials rather than panicking on them (TVmaze returns every season's
  episodes in one flat list, unlike TMDB's per-season endpoint).
- New TvMetadataSource setting (settings.rs), persisted alongside the TMDB
  token, defaulting to Tmdb for schema-evolution safety.
- metadata.rs now branches TV fetches through an AnyTvClient wrapper so the
  existing per-(series, season) caching loop doesn't care which source
  produced a result; the movie-fetch path is untouched.
- Frontend: TMDB/TVmaze picker in Fetch Data Settings, persisted on change;
  Fetch Data's token gating now only requires a TMDB token when it's actually
  needed (movies always, TV only when TMDB is the chosen source) instead of
  unconditionally blocking on it.

cargo test --workspace: 88/88 passing (81 + 7 new TVmaze + 3 new settings).
npm run build passing. Playwright verification: 18/18 checks (picker renders
and persists across reload, TV Fetch Data succeeds on TVmaze with zero TMDB
token configured, movies still correctly demand a token even with TVmaze
selected, switching back to TMDB still works end-to-end).

src-tauri/src/lib.rs

@@ -93,6 +93,8 @@ pub fn run() {
             metadata::fetch_movie_metadata,
             settings::get_tmdb_token,
             settings::save_tmdb_token,
+            settings::get_tv_metadata_source,
+            settings::save_tv_metadata_source,
             history::list_rename_history,
             history::undo_rename_batch,
             presets::list_presets,