Add OpenSubtitles subtitle search + download (new Subtitles panel)

New web client (crates/aurora-curator-core/src/web/opensubtitles.rs, 9 tests):
search, login, download, and fetch-subtitle-text against OpenSubtitles' REST
API. Search only needs a free app-level API key; download additionally needs
the end user's own OpenSubtitles account login (username/password -> JWT),
since downloads are quota-limited per account. Settings gained three new
fields (opensubtitles_api_key/username/password) following the existing
TMDB/OMDb credential pattern, with one deliberate exception: the password is
never returned to the frontend, only whether one is saved
(has_opensubtitles_password), since it's a real account password rather than
an API token.

New src-tauri/src/subtitles.rs wraps search/download as Tauri commands and
writes a downloaded subtitle right next to its video file
(Show.S01E01.mkv -> Show.S01E01.en.srt).

New src/components/SubtitlesPanel.tsx is the first real screen behind a
sidebar entry besides Rename (previously a stub) - load a video, auto-fill
the search form from the same local guess_episode/guess_movie Match uses,
search, and download with live remaining/allowed quota shown per result.

TheTVDB was considered instead but deliberately skipped for now: its v4 API
requires either a paid commercial license or a per-end-user paid TheTVDB
subscription just to log in, unlike every other source in this app.

144 tests passing (workspace). Updated README (status paragraph, project
layout, new "Subtitles" section, roadmap items 5/6/7).

src-tauri/src/lib.rs

@@ -4,6 +4,7 @@ mod history;
 mod metadata;
 mod presets;
 mod settings;
+mod subtitles;
 
 use aurora_curator_core::format::ExpressionFormat;
 use aurora_curator_core::matching::{self, EpisodeGuess, MovieGuess, ReleaseInfo};
@@ -107,6 +108,13 @@ pub fn run() {
             settings::save_tv_metadata_source,
             settings::get_movie_metadata_source,
             settings::save_movie_metadata_source,
+            settings::get_opensubtitles_api_key,
+            settings::save_opensubtitles_api_key,
+            settings::get_opensubtitles_username,
+            settings::has_opensubtitles_password,
+            settings::save_opensubtitles_login,
+            subtitles::search_subtitles,
+            subtitles::download_subtitle,
             history::list_rename_history,
             history::undo_rename_batch,
             presets::list_presets,