perf(feed): collapse count + facets into one /feed/summary pass (E2 S4, PERF-13)

The FE fired /feed/count (1 pass) + /facets (3 passes) on every filter change =
4 full scans of the subscribed set. New /api/feed/summary does ONE grouped query
(the base filter with tags stripped, GROUP BY channel_id -> per-channel video
counts) and derives the total count + every tag's distinct-channel facet count in
Python on the ~few-hundred-row channel map, faithfully mirroring filtered_query's
channel-based tag logic + the /facets drill-down (own category dropped, unless a
conjunctive topic-AND). Returns {count, facets} — the same numbers the two old
endpoints returned.

FE: a single shared query (Feed reads .count, Sidebar reads .facets); same key +
filters, so React Query dedupes to ONE request. Invalidations repointed to
qk.feedSummary. The old /feed/count + /facets endpoints (and their now-unused FE
api/qk) are kept as the equivalence-test oracle + safety net; a later cleanup can
drop them.

New equivalence test (test_feed_views, 8 filter combos incl. cross-category,
disjunctive/conjunctive topic modes, search): /feed/summary count == /feed/count
and facets == /facets, byte-identical. Structural win: 2 requests / 4-6 queries
-> 1 request / 2-3 queries (the review measured 530 -> 144 ms). Gate green:
344 pytest, frontend tsc/eslint/knip/prettier/222 vitest.

frontend/src/components/Sidebar.tsx

@@ -87,14 +87,16 @@ export default function Sidebar({
   // Live per-tag channel counts for the current filter context. Debounce the search term so
   // typing doesn't refire facets per keystroke, and keep previous counts during a refetch.
   const facetFilters: FeedFilters = { ...filters, q: useDebounced(filters.q, 300) };
-  const facetsQuery = useQuery({
-    queryKey: qk.facets(facetFilters),
-    queryFn: () => api.facets(facetFilters),
+  // Shared with the feed's count (same key + filters -> React Query dedupes to one request): the feed
+  // reads `.count`, this reads `.facets` (PERF-13, replaces the separate facets query).
+  const summaryQuery = useQuery({
+    queryKey: qk.feedSummary(facetFilters),
+    queryFn: () => api.feedSummary(facetFilters),
     staleTime: 30_000,
     placeholderData: keepPreviousData,
   });
-  const facetsReady = !!facetsQuery.data;
-  const facetCounts = facetsQuery.data?.counts ?? {};
+  const facetsReady = !!summaryQuery.data;
+  const facetCounts = summaryQuery.data?.facets ?? {};
   const chipCount = (tag: Tag): number =>
     facetsReady ? (facetCounts[String(tag.id)] ?? 0) : tag.channel_count;
   const visibleChips = (list: Tag[]): Tag[] => {