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  facets as "SUPERSEDED by /feed/summary — kept only as the equivalence oracle;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).

frontend/src/lib/api/feed.ts

@@ -113,6 +113,8 @@ function feedQuery(f: FeedFilters, cursor: string | null, limit: number): string
 export const feedApi = {
   feed: (f: FeedFilters, cursor: string | null, limit: number): Promise<FeedResponse> =>
     req(`/api/feed?${feedQuery(f, cursor, limit)}`),
+  // SUPERSEDED by feedSummary (PERF-13) — no component calls this anymore; the backend /feed/count
+  // endpoint it targets is kept only as the /feed/summary equivalence-test oracle. Retire together.
   feedCount: (f: FeedFilters): Promise<{ count: number }> =>
     req(`/api/feed/count?${filterParams(f).toString()}`),
   // Live YouTube search: results are materialised into the catalog and returned as feed cards.
@@ -129,6 +131,8 @@ export const feedApi = {
   // the now-orphaned, un-kept videos). Returns how many videos were removed.
   clearSearch: (videoIds: string[]): Promise<{ removed: number }> =>
     req("/api/search/clear", { method: "POST", body: JSON.stringify({ video_ids: videoIds }) }),
+  // SUPERSEDED by feedSummary (PERF-13) — no component calls this anymore; the backend /facets
+  // endpoint it targets is kept only as the /feed/summary equivalence-test oracle. Retire together.
   facets: (f: FeedFilters): Promise<{ counts: Record<string, number> }> =>
     req(`/api/facets?${filterParams(f).toString()}`),
   // The sidebar's total count + per-tag channel facets in ONE request + ONE grouped query, replacing