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>

.gitea/workflows/docker-publish.yml

@@ -7,15 +7,15 @@ on:
 
 env:
   # Number of tagged (non-latest) versions to keep per image name.
-  KEEP_VERSIONS: "5"
+  KEEP_VERSIONS: '5'
 
 jobs:
   build:
     runs-on: ubuntu-latest
     outputs:
-      host:  ${{ steps.meta.outputs.host }}
+      host: ${{ steps.meta.outputs.host }}
       image: ${{ steps.meta.outputs.image }}
-      sha:   ${{ steps.meta.outputs.sha }}
+      sha: ${{ steps.meta.outputs.sha }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4