Fix: ID gate missed Deezer-track-id-only rows

The original gate baked into `_select_discovery_tracks` only checked
Spotify + iTunes:

    AND (spotify_track_id IS NOT NULL OR itunes_track_id IS NOT NULL)

For Deezer-primary users, discovery_pool rows have populated
`deezer_track_id` but NULL Spotify + NULL iTunes IDs. The gate
filtered every row out — Time Machine, Genre Browser, Hidden Gems,
Discovery Shuffle, Popular Picks all rendered "no tracks found" for
every tab on every Deezer-primary install.

Extended the gate to include `deezer_track_id` and added that column
to the standard SELECT column tuple. `_build_track_dict` already
exposed `deezer_track_id` in its output shape, so frontend rendering
needed no changes.

Regression pinned via new test
`test_discovery_helper_accepts_deezer_only_id_rows` — inserts a row
with NULL Spotify + NULL iTunes but a populated `deezer_track_id`
and asserts it survives the gate.

2220/2220 full suite green.

core/personalized_playlists.py

@@ -111,6 +111,7 @@ class PersonalizedPlaylistsService:
     _STANDARD_DISCOVERY_COLUMNS: Tuple[str, ...] = (
         'spotify_track_id',
         'itunes_track_id',
+        'deezer_track_id',
         'track_name',
         'artist_name',
         'album_name',
@@ -141,7 +142,7 @@ class PersonalizedPlaylistsService:
 
         The WHERE clause always includes:
             source = ?
-            AND (spotify_track_id IS NOT NULL OR itunes_track_id IS NOT NULL)
+            AND (spotify_track_id IS NOT NULL OR itunes_track_id IS NOT NULL OR deezer_track_id IS NOT NULL)
             AND LOWER(artist_name) NOT IN
                 (SELECT LOWER(artist_name) FROM discovery_artist_blacklist)
 
@@ -181,7 +182,7 @@ class PersonalizedPlaylistsService:
                         {select_cols}
                 FROM discovery_pool
                 WHERE source = ?
-                  AND (spotify_track_id IS NOT NULL OR itunes_track_id IS NOT NULL)
+                  AND (spotify_track_id IS NOT NULL OR itunes_track_id IS NOT NULL OR deezer_track_id IS NOT NULL)
                   AND LOWER(artist_name) NOT IN (SELECT LOWER(artist_name) FROM discovery_artist_blacklist)
                   {extra_where}
                 ORDER BY {order_by}

tests/test_personalized_playlists_id_gate.py

@@ -135,6 +135,29 @@ def test_discovery_helper_filters_null_id_rows(service):
     assert names == ['Has Spotify ID', 'Has iTunes ID']
 
 
+def test_discovery_helper_accepts_deezer_only_id_rows(service):
+    """Discovery pool rows with NULL spotify + NULL itunes but a populated
+    deezer_track_id MUST pass the gate. Regression test — early version
+    of the gate only checked Spotify + iTunes, which silently filtered
+    out every row for Deezer-primary users (entire Time Machine /
+    Genre / Hidden Gems / Shuffle / Popular Picks rendered empty)."""
+    svc, db = service
+    db.insert_discovery_track(
+        source='deezer', deezer_track_id='dz1',
+        spotify_track_id=None, itunes_track_id=None,
+        track_name='Deezer Only', artist_name='A', album_name='X',
+        popularity=50, release_date='2024-01-01',
+    )
+    with patch.object(svc, '_get_active_source', return_value='deezer'):
+        tracks = svc._select_discovery_tracks(
+            source='deezer',
+            order_by='track_name',
+            fetch_limit=100,
+        )
+    assert [t['track_name'] for t in tracks] == ['Deezer Only']
+    assert tracks[0]['deezer_track_id'] == 'dz1'
+
+
 def test_discovery_helper_filters_blacklisted_artists(service):
     svc, db = service
     db.insert_discovery_track(