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/App.tsx

@@ -2,6 +2,7 @@ import { useState } from "react";
 import Sidebar, { PANELS } from "./components/Sidebar";
 import Header from "./components/Header";
 import RenamePanel from "./components/RenamePanel";
+import SubtitlesPanel from "./components/SubtitlesPanel";
 import PanelStub from "./components/PanelStub";
 
 export default function App() {
@@ -14,7 +15,13 @@ export default function App() {
       <div className="main-column">
         <Header title={current.name} />
         <div className="main-content">
-          {selected === "rename" ? <RenamePanel /> : <PanelStub name={current.name} />}
+          {selected === "rename" ? (
+            <RenamePanel />
+          ) : selected === "subtitles" ? (
+            <SubtitlesPanel />
+          ) : (
+            <PanelStub name={current.name} />
+          )}
         </div>
       </div>
     </div>