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/services/historyService.ts

@@ -2,16 +2,18 @@ import api from './api';
 import type { FeedbackAction, HistoryEntry } from '../types';
 
 export const historyService = {
-  // POST /api/history { trackId, completed?, batchId? } -> { historyId }
+  // POST /api/history { trackId, completed?, batchId?, listenedMs? } -> { historyId }
   async recordPlay(
     trackId: string,
     completed?: boolean,
-    batchId?: string
+    batchId?: string,
+    listenedMs?: number
   ): Promise<{ historyId: string }> {
     const res = await api.post<{ historyId: string }>('/history', {
       trackId,
       completed,
       batchId,
+      listenedMs,
     });
     return res.data;
   },