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/feedquery.py

@@ -113,15 +113,19 @@ def _tag_visible(user: User):
 MISSING_TAG_CATEGORY = "missing"
 
 
-def tags_by_category(db: Session, tag_ids: list[int]) -> dict[str, list[int]]:
+def tags_by_category(db: Session, user: User, tag_ids: list[int]) -> dict[str, list[int]]:
     """Group tag ids by their category (language/topic/other). The single source for the category split
     shared by the feed's tag WHERE (filtered_query) and the facet/count summary (feed_summary), so the
     two can't disagree on which selected tag belongs to which category. An unknown id lands in
-    MISSING_TAG_CATEGORY."""
+    MISSING_TAG_CATEGORY — and so does another user's private tag: resolving it to its real category
+    told the caller which foreign ids exist (E32 review)."""
     if not tag_ids:
         return {}  # skip the pointless `WHERE id IN ()` round-trip on the no-tag-filter hot path
     out: dict[str, list[int]] = {}
-    for tid, cat in db.execute(select(Tag.id, Tag.category).where(Tag.id.in_(tag_ids))).all():
+    visible = or_(Tag.user_id.is_(None), Tag.user_id == user.id)
+    for tid, cat in db.execute(
+        select(Tag.id, Tag.category).where(Tag.id.in_(tag_ids), visible)
+    ).all():
         out.setdefault(cat, []).append(tid)
     known = {tid for ids in out.values() for tid in ids}
     if missing := [tid for tid in tag_ids if tid not in known]:
@@ -315,7 +319,7 @@ def filtered_query(
         # ALL its selected tags. ⚠️ feed_summary.passes() reimplements this exact rule in Python for the
         # sidebar count/facets — keep the two in lockstep (equivalence-tested by
         # test_feed_summary_matches_count_plus_facets).
-        by_category = tags_by_category(db, tags)
+        by_category = tags_by_category(db, user, tags)
         # Facet counting drops the category being counted so its own chips don't zero each
         # other out (standard drill-down faceting: a category's count ignores its own
         # selections but still honours the other categories' filters).
@@ -406,7 +410,7 @@ def feed_summary(db: Session, user: User, **params) -> dict:
         return {"count": 0, "facets": {}}
 
     # channel -> its visible tag ids, plus each tag's category (for grouping the facet output).
-    visible = or_(ChannelTag.user_id.is_(None), ChannelTag.user_id == user.id)
+    visible = _tag_visible(user)
     channel_tags: dict[str, set[int]] = {}
     tag_category: dict[int, str] = {}
     for cid, tid, cat in db.execute(
@@ -421,7 +425,7 @@ def feed_summary(db: Session, user: User, **params) -> dict:
     # base channel carries still filters everything out for its category — matching the SQL `IN (empty)`
     # behaviour).
     selected_by_cat: dict[str, set[int]] = {
-        cat: set(ids) for cat, ids in tags_by_category(db, params.get("tags") or []).items()
+        cat: set(ids) for cat, ids in tags_by_category(db, user, params.get("tags") or []).items()
     }
 
     def passes(ch_tags: set[int], exclude_cat: str | None) -> bool: