feat(discover): wire A4 search + A5 downloads to backend

Adds DownloadJob/ExternalSearchResult/SourceInfo contract types + mappers, the downloads + search RTKQ endpoints, and the SearchDownloadPage (search external sources, per-result download states) and DownloadsManagerPage (active/history, progress, retry/cancel, poll-while-active). en/ru i18n. Snapshot also bundles in-progress queue/metadata-editor/storage UI work.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/types.ts

@@ -78,20 +78,62 @@ export interface PlaylistTrack extends Track {
   addedAt: string;
 }
 
+/** Lifecycle of a download job, mirroring the backend's `DownloadStatus`.
+ * `enriching` = file fetched, metadata pipeline running; `done` = imported. */
+export type DownloadStatus =
+  | 'queued'
+  | 'downloading'
+  | 'enriching'
+  | 'done'
+  | 'failed';
+
 export interface DownloadJob {
   id: string;
-  url: string;
-  title?: string;
-  artist?: string;
-  album?: string;
-  status: 'queued' | 'downloading' | 'processing' | 'done' | 'error';
+  /** Source backend the job pulls from (e.g. `youtube`). */
+  source: string;
+  /** Stable per-source id of the item (e.g. a YouTube videoId). */
+  sourceId?: string;
+  /** The free-text query the job was created from, for display. */
+  query?: string;
+  status: DownloadStatus;
+  /** Fraction complete, 0..1. */
   progress: number;
   errorMessage?: string;
+  /** Set once the download finishes and the library track exists. */
   trackId?: string;
+  retryCount: number;
   createdAt: string;
   updatedAt: string;
 }
 
+/** Result of POST /downloads. Either the item is already in the library
+ * (`alreadyInLibrary`, `trackId` set), or a job covers it (`job`). */
+export interface DownloadRequestResult {
+  alreadyInLibrary: boolean;
+  trackId?: string;
+  job?: DownloadJob;
+}
+
+/** One hit from an external (fetch) source — the §A4 discover screen. */
+export interface ExternalSearchResult {
+  source: string;
+  sourceId: string;
+  title: string;
+  artist?: string;
+  album?: string;
+  durationMs?: number;
+  thumbnailUrl?: string;
+}
+
+/** A registered source backend, from GET /sources. `kind`: `indexable` (a
+ * mounted folder) or `fetch` (searchable + downloadable, e.g. YouTube). */
+export interface SourceInfo {
+  name: string;
+  label: string;
+  kind: 'indexable' | 'fetch';
+  available: boolean;
+}
+
 export interface UploadResponse {
   track_id: string;
   title: string;