perf(feed): skip the empty IN() query in tags_by_category (E2 review round 2)

The tags_by_category extraction dropped feed_summary's `if selected:` guard, so
a /feed/summary request with no tag filter (the common case) ran a pointless
`SELECT ... WHERE id IN ()` round-trip. Early-return {} for empty tag_ids —
restores the no-query hot path and covers filtered_query + future callers too.
Gate green (346 pytest).

backend/app/feedquery.py

@@ -104,6 +104,8 @@ def tags_by_category(db: Session, 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."""
+    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():
         out.setdefault(cat, []).append(tid)