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>

backend/src/services/db.service.ts

@@ -69,6 +69,22 @@ import type {
 import { AUDIO_PREFERENCE_BUCKETS, FEEDBACK_ACTIONS } from '../db/types.js';
 export * from '../db/types.js';
 
+/**
+ * Every play_history write goes through this statement.
+ *
+ * The title/artist copy is taken in the same round trip via LEFT JOIN, so a
+ * play stays readable after its track is hard-deleted, and the join never
+ * suppresses the insert when the track id is unknown.
+ */
+const PLAY_HISTORY_INSERT = `
+  INSERT INTO play_history (
+    user_id, track_id, batch_id, completed, listened_ms, track_title, track_artist
+  )
+  SELECT $1::uuid, $2::uuid, $3::uuid, $4::boolean, $5::int, t.title, t.artist
+    FROM (SELECT 1) AS always
+    LEFT JOIN tracks t ON t.id = $2::uuid
+  RETURNING id`;
+
 export class DbService {
   /** Exposed so route handlers (e.g. settings) can query the database directly. */
   readonly pgClient: Pool;
@@ -483,11 +499,17 @@ export class DbService {
   // (permanentlyDeleteTrack, below) and exactly one unlink site in the whole
   // system (workers/src/cleanup.service.ts).
 
-  async recordPlay(userId: string, trackId: string, completed: boolean, batchId?: string): Promise<string> {
+  async recordPlay(
+    userId: string,
+    trackId: string,
+    completed: boolean,
+    batchId?: string,
+    listenedMs?: number,
+  ): Promise<string> {
     if (!completed) {
       const res = await this.pgClient.query(
-        'INSERT INTO play_history (user_id, track_id, batch_id, completed) VALUES ($1, $2, $3, $4) RETURNING id',
-        [userId, trackId, batchId ?? null, false]
+        PLAY_HISTORY_INSERT,
+        [userId, trackId, batchId ?? null, false, listenedMs ?? null]
       );
       return res.rows[0].id as string;
     }
@@ -497,8 +519,8 @@ export class DbService {
     return this.withTransaction(async (client) => {
       // 1. Record play history
       const insertRes = await client.query(
-        'INSERT INTO play_history (user_id, track_id, batch_id, completed) VALUES ($1, $2, $3, $4) RETURNING id',
-        [userId, trackId, batchId ?? null, true]
+        PLAY_HISTORY_INSERT,
+        [userId, trackId, batchId ?? null, true, listenedMs ?? null]
       );
       const historyId = insertRes.rows[0].id as string;
 
@@ -1936,10 +1958,17 @@ export class DbService {
     const occurredAt = event.occurred_at?.toISOString?.() ?? new Date().toISOString();
     switch (event.type) {
       case 'completed':
+        // The Vibe player is the one caller that knows how much was actually
+        // heard. position_ms is where playback reached; duration_ms is the
+        // fallback for a client that only reported the track length.
         await client.query(
-          `INSERT INTO play_history (user_id, track_id, completed, played_at)
-           VALUES ($1, $2, true, $3::timestamptz)`,
-          [event.user_id, event.track_id, occurredAt],
+          `INSERT INTO play_history (
+             user_id, track_id, completed, played_at, listened_ms, track_title, track_artist
+           )
+           SELECT $1::uuid, $2::uuid, true, $3::timestamptz, $4::int, t.title, t.artist
+             FROM (SELECT 1) AS always
+             LEFT JOIN tracks t ON t.id = $2::uuid`,
+          [event.user_id, event.track_id, occurredAt, event.position_ms ?? event.duration_ms ?? null],
         );
         await client.query(
           `UPDATE tracks