refactor(feed): address /code-review high findings on the E2 stack

Review round on the E2 stack (S1-S5). No correctness bugs; the surviving
low/medium findings addressed:

- altitude (#1): extract the shared `tags_by_category` helper used by BOTH
  filtered_query (SQL tag WHERE) and feed_summary (Python facet/count), removing
  the category-grouping duplication, and add bidirectional "keep in lockstep —
  equivalence-tested by test_feed_summary_matches_count_plus_facets" comments so
  the SQL<->Python tag rule can't silently drift.
- test-coverage (#2): the /feed/summary equivalence test seeded only system tags,
  leaving the `ChannelTag.user_id == user.id` visibility branch untested. Add a
  user-owned Tag + link + two combos; the suite is now 346 (was 344) and the new
  cases pass (feed_summary agrees with /feed/count + /facets on user-visible tags).
- dead-code (#3): annotate get_feed_count, get_facets, and the FE api.feedCount/
  facets as "SUPERSEDED by /feed/summary — kept only as the equivalence oracle;
  retire together" so the intentionally-retained surface reads as deliberate.

Not changed (confirmed no-op): the search channel-name resolve (#4) is
once-per-request on a 633-row table and a deliberate pre-resolve for PERF-12's
BitmapOr plan; the facet loop (#5) already computes passes() once per
(channel, category) with a distinct exclude each, so there is no recomputation.

Gate green: ruff + 346 pytest; frontend tsc/knip/prettier (FE change is a comment).

backend/app/routes/feed.py

@@ -128,6 +128,9 @@ def get_feed_count(
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
+    # SUPERSEDED by /feed/summary (PERF-13) — the app no longer calls this. Kept as the reference
+    # oracle that test_feed_summary_matches_count_plus_facets asserts /feed/summary's `count` against;
+    # remove both together with that test if this endpoint is ever retired.
     query, _rank = filtered_query(db, user, **params)
     total = db.scalar(select(func.count()).select_from(query.subquery()))
     return {"count": total or 0}
@@ -147,7 +150,11 @@ def get_facets(
 
     The count is the number of distinct channels that have at least one video in the current
     view, matching the existing channel-count chip semantics. JSON object keys are strings
-    (tag ids)."""
+    (tag ids).
+
+    SUPERSEDED by /feed/summary (PERF-13) — the app no longer calls this. Kept as the reference
+    oracle for test_feed_summary_matches_count_plus_facets (it asserts /feed/summary's `facets`
+    against these `counts`); retire the two together."""
     visible = or_(ChannelTag.user_id.is_(None), ChannelTag.user_id == user.id)
     counts: dict[int, int] = {}
     for category in ("language", "topic", "other"):