fix(#945): Spotify backfill must disable cross-service search fallback

Double-checking the backfill logic found a real correctness bug. Spotify search_tracks defaults to
allow_fallback=True, so when Spotify is rate-limited or in free mode it returns iTunes/Deezer tracks
whose .id is an iTunes/Deezer id, NOT a Spotify id. The backfill took that .id as a Spotify track id
and would push wrong/garbage tracks into the exported Spotify playlist. The unit tests used fake
Track objects with hand-set ids, so they could never surface this cross-service contamination.

Fix: the Spotify backfill search now passes allow_fallback=False — real Spotify hits or nothing
(an unmatched track is left out, never replaced by a non-Spotify id). Deezer is unaffected: its
search fallback is query-only and stays within Deezer, so its ids are always Deezer ids.

Regression test asserts the Spotify backfill search is invoked with allow_fallback=False. 8
orchestration tests green, ruff clean.

tests/test_service_playlist_export.py

@@ -110,3 +110,21 @@ def test_backfill_on_wires_search_fn(monkeypatch):
                            _FakeClient({'success': True, 'playlist_id': 'p', 'added': 1}),
                            _fake_resolver(['sx'], seen))
     assert seen['search_id_fn'] == 'SEARCH_FN'
+
+
+def test_spotify_backfill_search_disables_cross_service_fallback(monkeypatch):
+    """REGRESSION: Spotify's search_tracks falls back to iTunes/Deezer (non-Spotify ids) under
+    rate-limit/free. The backfill MUST disable that or it pushes wrong ids into the Spotify
+    playlist. Assert the search is invoked with allow_fallback=False."""
+    seen = {}
+
+    class _FakeSpotify:
+        def search_tracks(self, q, limit=10, allow_fallback=True):
+            seen['allow_fallback'] = allow_fallback
+            return []
+
+    monkeypatch.setattr(ws, 'get_spotify_client', lambda: _FakeSpotify())
+    fn = ws._build_service_search_id_fn('spotify')
+    assert fn is not None
+    fn('Kendrick Lamar', 'Not Like Us')   # drives the search
+    assert seen['allow_fallback'] is False

web_server.py

@@ -27749,12 +27749,21 @@ def _build_service_search_id_fn(service):
     try:
         if service == 'spotify':
             client = get_spotify_client()
+            if not client:
+                return None
+            # allow_fallback=False is CRITICAL: Spotify's search falls back to iTunes/Deezer
+            # when rate-limited/free, returning tracks whose .id is NOT a Spotify id — backfill
+            # would then push wrong ids into the Spotify playlist. Disable it: real Spotify hits
+            # or nothing.
+            search_fn = lambda q: client.search_tracks(q, limit=8, allow_fallback=False)  # noqa: E731
         else:
             from core.metadata.registry import get_deezer_client
             client = get_deezer_client()
-        if not client:
-            return None
-        search_fn = lambda q: client.search_tracks(q, limit=8)  # noqa: E731
+            if not client:
+                return None
+            # Deezer's search stays within Deezer (query-only fallback), so its .id is always a
+            # Deezer id — no cross-service guard needed.
+            search_fn = lambda q: client.search_tracks(q, limit=8)  # noqa: E731
         return lambda artist, title: search_service_track_id(artist, title, search_fn=search_fn)
     except Exception:
         return None