fix(autotag): the orphan cleanup keeps a system tag a saved view filters on

Feed views store tag ids. A system tag no channel carried for a while was deleted and, once a
channel earned it again, recreated under a new id — the view's filter then pointed at nothing, the
feed dropped the unknown id, and the view showed the whole feed under its old name. The cleanup now
skips tags a saved feed view references (test seen red without it).

backend/tests/test_autotag_orphans.py

@@ -0,0 +1,36 @@
+"""sync/autotag's orphan cleanup (E32 S2: the module was 32% covered).
+
+A system tag no channel carries any more is deleted — unless a saved feed view filters on it. The
+views hold tag ids; a deleted tag returns under a NEW id when a channel earns it again, and the feed
+drops an unknown id, so the view silently showed the whole feed under its old name.
+"""
+
+from sqlalchemy import select
+
+from app.models import Channel, ChannelTag, SavedView, Tag
+from app.sync import autotag
+
+
+def test_an_orphan_system_tag_goes_but_one_a_saved_view_filters_on_stays(db, make_user):
+    user = make_user()
+    used = Tag(user_id=None, name="Gaming", category="topic")
+    orphan = Tag(user_id=None, name="Cooking", category="topic")
+    in_view = Tag(user_id=None, name="Music", category="topic")
+    own = Tag(user_id=user.id, name="Mine", category="other")
+    ch = Channel(id="UC_tags", title="T")
+    db.add_all([used, orphan, in_view, own, ch])
+    db.flush()
+    db.add(ChannelTag(channel_id=ch.id, tag_id=used.id, user_id=None, source="auto"))
+    db.add(
+        SavedView(
+            user_id=user.id, source="feed", name="Music", filters={"tags": [in_view.id], "q": ""}
+        )
+    )
+    # A Plex view's filters are another vocabulary; they must not break the scan.
+    db.add(SavedView(user_id=user.id, source="plex", name="P", filters={"genres": ["Drama"]}))
+    db.commit()
+
+    assert autotag._cleanup_orphan_system_tags(db) == 1
+    db.expire_all()
+    names = set(db.execute(select(Tag.name)).scalars())
+    assert names == {"Gaming", "Music", "Mine"}  # only the unreferenced system orphan went