Test the REAL Plex/Jellyfin sync matcher's durable-match fallback (#895)

Closes the gap from cf2b4d7d: that commit unit-tested the DB-only matcher twin but
not services.sync_service._find_track_in_media_server — the actual path a connected
Plex/Jellyfin user hits. Drive it directly (Jellyfin server-type, so no Plex fetchItem
mocking): durable manual match is honored when the volatile cache is wiped, and a
stale library id self-heals via the stored file path.

tests/sync/test_sync_durable_match.py

@@ -0,0 +1,57 @@
+"""The REAL playlist sync matcher (PlaylistSyncService._find_track_in_media_server)
+must honor a durable Find & Add / manual match when the volatile sync_match_cache
+has been wiped by a library rescan — otherwise the manual pick is re-matched from
+scratch on the next auto-sync (#895 follow-up). Jellyfin server-type avoids Plex
+fetchItem mocking; the durable block is server-agnostic."""
+
+from __future__ import annotations
+
+import asyncio
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+from services.sync_service import PlaylistSyncService
+
+
+def _service():
+    svc = PlaylistSyncService(spotify_client=MagicMock(), download_orchestrator=MagicMock(),
+                              media_server_engine=MagicMock())
+    client = MagicMock(); client.is_connected.return_value = True
+    svc._get_active_media_client = lambda: (client, "jellyfin")
+    svc._cancelled = False
+    return svc
+
+
+def _run(svc, db):
+    cm = MagicMock(); cm.get_active_media_server.return_value = "jellyfin"
+    track = SimpleNamespace(name="Valió la Pena - Salsa Version", artists=["Marc Anthony"], id="sp16")
+    with patch("database.music_database.MusicDatabase", return_value=db), \
+         patch("config.settings.config_manager", cm), \
+         patch("core.artists.map.get_current_profile_id", return_value=1):
+        return asyncio.run(svc._find_track_in_media_server(track, candidate_pool={}))
+
+
+def test_durable_match_used_when_volatile_cache_wiped():
+    dt = SimpleNamespace(id="t99", title="Valió la Pena (Salsa Version)")
+    db = MagicMock()
+    db.read_sync_match_cache.return_value = None                 # rescan wiped the cache
+    db.check_track_exists.return_value = (None, 0.0)             # fuzzy would FAIL
+    db.find_manual_library_match_by_source_track_id.return_value = {
+        "library_track_id": "t99", "library_file_path": "/m/x.flac"}
+    db.get_track_by_id.side_effect = lambda i: dt if str(i) == "t99" else None
+    match, conf = _run(svc=_service(), db=db)
+    assert conf == 1.0 and match.id == "t99"                    # manual pick honored across the rescan
+
+
+def test_durable_match_self_heals_stale_library_id():
+    dt = SimpleNamespace(id="newid", title="X")
+    db = MagicMock()
+    db.read_sync_match_cache.return_value = None
+    db.check_track_exists.return_value = (None, 0.0)
+    db.find_manual_library_match_by_source_track_id.return_value = {
+        "library_track_id": "staleid", "library_file_path": "/m/x.flac"}
+    db.get_track_by_id.side_effect = lambda i: dt if str(i) == "newid" else None  # stale id gone
+    db.find_track_id_by_file_path.return_value = "newid"
+    match, conf = _run(svc=_service(), db=db)
+    assert conf == 1.0 and match.id == "newid"
+    db.find_track_id_by_file_path.assert_called_once_with("/m/x.flac")