Sync: append mode preserves the playlist image like reconcile (#811)

#811 (reopen of #792, carlosjfcasero, Emby/Jellyfin): "append" mode clobbered
the playlist's custom image. The post-sync image push only excluded
'reconcile' — so append (which edits the playlist in place via
append_to_playlist) still re-pushed the source image over the user's poster
every sync. Now both in-place modes (reconcile + append) skip the image push;
only the destructive 'replace' (recreate-from-scratch) pushes it.

append_to_playlist + set_playlist_image were verified to NOT touch tracks or
description (image push only POSTs /Images/Primary), so this is the identity-
clobber fix for append.

Tests: append + reconcile preserve the image, replace still pushes it.

core/discovery/sync.py

@@ -524,12 +524,12 @@ def run_sync_task(
         # don't want persisted to app.log.
         _synced = getattr(result, 'synced_tracks', 0)
         logger.info(f"[PLAYLIST IMAGE] has_image={bool(playlist_image_url)}, synced_tracks={_synced}")
-        # In reconcile mode the whole point (#792) is to NOT touch the playlist's
-        # existing image — pushing the source image here would re-clobber a
-        # user's custom poster every sync, the exact bug reconcile fixes. So skip
-        # the auto image push for reconcile (the playlist keeps its own art).
-        if sync_mode == 'reconcile':
-            logger.info("[PLAYLIST IMAGE] reconcile mode — preserving existing playlist image")
+        # Modes that edit a playlist in place (reconcile #792, append #811) must
+        # NOT push the source image — doing so re-clobbers a user's custom poster
+        # every sync, the exact bug these modes exist to avoid. Only the
+        # destructive 'replace' (recreate-from-scratch) pushes the image.
+        if sync_mode in ('reconcile', 'append'):
+            logger.info(f"[PLAYLIST IMAGE] {sync_mode} mode — preserving existing playlist image")
         elif playlist_image_url and _synced > 0:
             try:
                 active_server = deps.config_manager.get_active_media_server()

tests/discovery/test_discovery_sync.py

@@ -364,6 +364,49 @@ def test_playlist_image_uploaded_to_jellyfin(patched_db):
     assert jf.image_calls == [('PJF', 'https://img/y.png')]
 
 
+def test_append_mode_preserves_playlist_image(patched_db):
+    """Append edits in place — it must NOT re-push the source image over the
+    user's custom poster (#811)."""
+    jf = _FakeJellyfin()
+    cfg = _FakeConfig(server='jellyfin')
+    result = _FakeSyncResult(synced_tracks=3)
+    svc = _FakeSyncService(media_client=_FakeMediaClient(), sync_result=result)
+    deps = _build_deps(sync_service=svc, jellyfin=jf, config=cfg)
+
+    ds.run_sync_task('pA', 'PA', [_track()],
+                     playlist_image_url='https://img/a.png', deps=deps, sync_mode='append')
+
+    assert jf.image_calls == []   # preserved, not clobbered
+
+
+def test_reconcile_mode_preserves_playlist_image(patched_db):
+    """Reconcile likewise preserves the image (#792)."""
+    jf = _FakeJellyfin()
+    cfg = _FakeConfig(server='jellyfin')
+    result = _FakeSyncResult(synced_tracks=3)
+    svc = _FakeSyncService(media_client=_FakeMediaClient(), sync_result=result)
+    deps = _build_deps(sync_service=svc, jellyfin=jf, config=cfg)
+
+    ds.run_sync_task('pR', 'PR', [_track()],
+                     playlist_image_url='https://img/r.png', deps=deps, sync_mode='reconcile')
+
+    assert jf.image_calls == []
+
+
+def test_replace_mode_still_pushes_playlist_image(patched_db):
+    """Replace recreates from scratch, so it does push the source image."""
+    jf = _FakeJellyfin()
+    cfg = _FakeConfig(server='jellyfin')
+    result = _FakeSyncResult(synced_tracks=3)
+    svc = _FakeSyncService(media_client=_FakeMediaClient(), sync_result=result)
+    deps = _build_deps(sync_service=svc, jellyfin=jf, config=cfg)
+
+    ds.run_sync_task('pRep', 'PRep', [_track()],
+                     playlist_image_url='https://img/rep.png', deps=deps, sync_mode='replace')
+
+    assert jf.image_calls == [('PRep', 'https://img/rep.png')]
+
+
 def test_no_image_upload_when_zero_synced(patched_db):
     """synced_tracks == 0 → no playlist image upload."""
     plex = _FakePlex()