Add OMDb as a datasource: TV episodes (third option alongside TMDB/TVmaze) and movies (new second option alongside TMDB)

Movies get their own independent metadata-source setting for the first
time (previously hardcoded to TMDB) so the GUI's Fetch Data Settings now
has two separate pickers - TV episode source (TMDB/TVmaze/OMDb) and
movie source (TMDB/OMDb) - and a batch can freely mix sources across its
TV and movie halves. The -rename CLI's --db flag gained omdb alongside
tmdb/tvmaze, with movies now looked up online for both tmdb and omdb.

src-tauri/src/lib.rs

@@ -101,8 +101,12 @@ pub fn run() {
             metadata::fetch_movie_metadata,
             settings::get_tmdb_token,
             settings::save_tmdb_token,
+            settings::get_omdb_api_key,
+            settings::save_omdb_api_key,
             settings::get_tv_metadata_source,
             settings::save_tv_metadata_source,
+            settings::get_movie_metadata_source,
+            settings::save_movie_metadata_source,
             history::list_rename_history,
             history::undo_rename_batch,
             presets::list_presets,