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>

tests/rtkqPersist.test.ts

@@ -20,7 +20,13 @@ beforeEach(() => {
 
 function apiStateWith(queries: Record<string, unknown>) {
   return {
-    api: { queries, mutations: {}, provided: {}, subscriptions: {}, config: {} },
+    api: {
+      queries,
+      mutations: {},
+      provided: {},
+      subscriptions: {},
+      config: {},
+    },
   } as unknown as RootState;
 }