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/app/sync/autotag.py

@@ -12,7 +12,7 @@ from sqlalchemy import and_, exists, func, select
 from sqlalchemy.orm import Session
 
 from app import logs, progress, sysconfig
-from app.models import Channel, ChannelTag, Tag, Video
+from app.models import Channel, ChannelTag, SavedView, Tag, Video
 
 log = logs.get_logger(__name__)
 
@@ -289,12 +289,22 @@ def run_autotag_all(db: Session, only_missing: bool = False) -> dict:
 
 
 def _cleanup_orphan_system_tags(db: Session) -> int:
-    """Delete system tags that no longer link to any channel (e.g. after a mapping change)."""
+    """Delete system tags that no longer link to any channel (e.g. after a mapping change) — but not
+    one a saved feed view filters on. Views hold tag IDS, and a deleted tag comes back under a new id
+    when a channel earns it again, so the view's filter would point at nothing; the feed drops an
+    unknown id, and the view showed the whole feed under its old name (E32 S2)."""
+    in_views = {
+        t
+        for (filters,) in db.execute(select(SavedView.filters).where(SavedView.source == "feed"))
+        for t in ((filters or {}).get("tags") or [])
+        if isinstance(t, int)
+    }
     orphans = (
         db.execute(
             select(Tag).where(
                 Tag.user_id.is_(None),
                 ~exists().where(ChannelTag.tag_id == Tag.id),
+                Tag.id.notin_(in_views),
             )
         )
         .scalars()