fix(feed): a malformed saved view cannot break the autotag cleanup, and a foreign private tag reads as missing

E32 review round 1 (B5, B6).

- The autotag orphan cleanup reads every user's feed views. A view whose tag pick was not a list
  ("tags": 5) raised for everyone's job, True passed as tag id 1, and ~66k ids pushed the NOT IN
  past psycopg's 65535 bind parameters. The views are now read defensively and the kept ids
  filtered in Python; the saved-views route holds filters.tags to a list of at most 500 ids, on
  create and on update.
- tags_by_category resolved any tag id, so another user's private tag landed in its real category
  and the feed summary answered differently than for an id that does not exist, which told the
  caller which foreign ids exist. It is owner-scoped now (pre-existing; S2's missing-tag branch
  made the difference visible).

backend/app/sync/autotag.py

@@ -288,28 +288,35 @@ def run_autotag_all(db: Session, only_missing: bool = False) -> dict:
     return {"channels_tagged": tagged, "orphan_tags_removed": removed}
 
 
+def _view_tag_ids(filters: object) -> list[int]:
+    """The tag ids a stored view's filters name; anything else in that slot names none."""
+    tags = filters.get("tags") if isinstance(filters, dict) else None
+    if not isinstance(tags, list):
+        return []
+    return [t for t in tags if type(t) is int]  # `type`, not isinstance: True is not a tag id
+
+
 def _cleanup_orphan_system_tags(db: Session) -> int:
     """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)."""
+    unknown id, and the view showed the whole feed under its old name (E32 S2).
+
+    The views are read defensively, and the kept ids are filtered here rather than in SQL: a view
+    stored before its route checked the shape (or written around it) must not raise for everyone's
+    job, nor push a `NOT IN` past the driver's bind-parameter limit (E32 review)."""
     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)
+        for t in _view_tag_ids(filters)
     }
-    orphans = (
-        db.execute(
-            select(Tag).where(
-                Tag.user_id.is_(None),
-                ~exists().where(ChannelTag.tag_id == Tag.id),
-                Tag.id.notin_(in_views),
-            )
-        )
-        .scalars()
-        .all()
-    )
+    orphans = [
+        tag
+        for tag in db.execute(
+            select(Tag).where(Tag.user_id.is_(None), ~exists().where(ChannelTag.tag_id == Tag.id))
+        ).scalars()
+        if tag.id not in in_views
+    ]
     for tag in orphans:
         db.delete(tag)
     db.commit()