feat(discovery): acquire recommendations that keep their names

Acquisition ran yt-dlp without --embed-metadata, so every download
arrived untagged. The scanner then stored the video id as the title and
"Unknown Artist" as the artist, the vetted-candidate tag check rejected
the mismatch, and all 18 acquired tracks were hidden and retired.

- Pass --embed-metadata so downloads carry real tags.
- Let a scan take fallback title/artist from the candidate, for sources
  that still ship untagged files.
- Install Deno alongside yt-dlp: YouTube guards some formats with a JS
  challenge yt-dlp must execute, and no other runtime is enabled.
- Dedupe candidates by artist and title. The (source, external_id) key
  misses the same song reaching us under two Deezer release ids.

Also carries the in-flight discovery work this builds on: the
Recommendations page replacing Discover, the discovery source service,
and the acquisition spec tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/components/NavRail.tsx

@@ -35,7 +35,7 @@ const NAV_GROUPS: NavGroup[] = [
     label: 'AI',
     items: [
       { to: '/vibe', icon: Zap, label: 'Vibe' },
-      { to: '/discover', icon: Compass, label: 'Discover' },
+      { to: '/recommendations', icon: Compass, label: 'Found' },
     ],
   },
   {