refactor(fe): query-key factory + migrate feed/channels/playlists (R7 S2a)

Introduce lib/queryKeys.ts as the single source of truth for React Query
keys — the R7 driver was ~200 hand-written key literals across 51 files
(`["feed"]` alone in 15+), where one typo silently orphans a key so an
invalidate never matches.

S2a migrates the three highest-traffic domains (feed/video, channels,
playlists) plus the shared me/my-status/tags roots: ~90 literal sites in
18 files now call qk.*(). Pure substitution — each factory fn returns the
byte-identical array the call sites used before, so TanStack's prefix
matching is unchanged (qk.feed() still invalidates qk.feed(filters)).
Co-invalidation clusters are deliberately NOT bundled into helpers (they
differ per site), so no query's refetch scope changes.

Nullable keys (playlist(selectedId), ytSearch(q)) accept null so a null
segment is preserved (["playlist", null]) while a no-arg call is the bare
prefix key — guard is `!== undefined`.

Gate green: typecheck, eslint (0 err), prettier, 68 vitest. Smoke: app
boots, feed/facets/tags/my-status all 200, no console errors. Factory
extended to plex/messaging/admin/downloads/notifications in S2b.

frontend/src/components/Feed.tsx

@@ -6,6 +6,7 @@ import {
   useQuery,
   useQueryClient,
 } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import {
   ArrowDown,
   ArrowUp,
@@ -155,7 +156,7 @@ export default function Feed({
   const queryFilters: FeedFilters = { ...filters, q: debouncedQ };
 
   const query = useInfiniteQuery({
-    queryKey: ["feed", queryFilters],
+    queryKey: qk.feed(queryFilters),
     queryFn: ({ pageParam }) => api.feed(queryFilters, pageParam as string | null, PAGE),
     initialPageParam: null as string | null,
     getNextPageParam: (last) => last.next_cursor ?? undefined,
@@ -168,7 +169,7 @@ export default function Feed({
   // retry:false so a 429 (quota/limit) surfaces at once for inline display. staleTime keeps a
   // revisited search from re-spending.
   const ytQuery = useInfiniteQuery({
-    queryKey: ["yt-search", ytSearch, ytCount],
+    queryKey: qk.ytSearch(ytSearch, ytCount),
     queryFn: ({ pageParam }) =>
       api.searchYoutube(ytSearch as string, pageParam as string | null, ytCount),
     initialPageParam: null as string | null,
@@ -225,7 +226,7 @@ export default function Feed({
   }, [query.dataUpdatedAt]);
 
   const countQuery = useQuery({
-    queryKey: ["feed-count", queryFilters],
+    queryKey: qk.feedCount(queryFilters),
     queryFn: () => api.feedCount(queryFilters),
     staleTime: 30_000,
     enabled: !ytActive,
@@ -250,7 +251,7 @@ export default function Feed({
       api
         .setState(id, status)
         .then(() => {
-          qc.invalidateQueries({ queryKey: ["feed"] });
+          qc.invalidateQueries({ queryKey: qk.feed() });
           if (status === "hidden") {
             const v = loadedRef.current.find((x) => x.id === id);
             notify({
@@ -307,7 +308,7 @@ export default function Feed({
       setOverrides((o) => ({ ...o, [id]: "new" }));
       api
         .clearState(id)
-        .then(() => qc.invalidateQueries({ queryKey: ["feed"] }))
+        .then(() => qc.invalidateQueries({ queryKey: qk.feed() }))
         .catch(() =>
           setOverrides((o) => {
             const next = { ...o };
@@ -324,8 +325,8 @@ export default function Feed({
       setSavedOverrides((o) => ({ ...o, [id]: saved }));
       (saved ? api.watchLaterAdd(id) : api.watchLaterRemove(id))
         .then(() => {
-          qc.invalidateQueries({ queryKey: ["playlists"] });
-          qc.invalidateQueries({ queryKey: ["playlist"] });
+          qc.invalidateQueries({ queryKey: qk.playlists() });
+          qc.invalidateQueries({ queryKey: qk.playlist() });
         })
         .catch(() => setSavedOverrides((o) => ({ ...o, [id]: !saved })));
     },
@@ -355,9 +356,9 @@ export default function Feed({
   // A live search ingests new catalog videos, so the disabled feed queries still hold their stale
   // pre-search cache. Drop them so the feed re-fetches fresh on return.
   const dropFeedCaches = () => {
-    qc.removeQueries({ queryKey: ["feed"] });
-    qc.removeQueries({ queryKey: ["feed-count"] });
-    qc.removeQueries({ queryKey: ["facets"] });
+    qc.removeQueries({ queryKey: qk.feed() });
+    qc.removeQueries({ queryKey: qk.feedCount() });
+    qc.removeQueries({ queryKey: qk.facets() });
   };
 
   // --- Live YouTube search mode -------------------------------------------------------------
@@ -435,7 +436,7 @@ export default function Feed({
                   setFilters({ ...filters, librarySource: "organic", sort: "newest" });
                   onExitYtSearch();
                   dropFeedCaches();
-                  qc.removeQueries({ queryKey: ["yt-search"] });
+                  qc.removeQueries({ queryKey: qk.ytSearch() });
                 }}
                 className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full border border-border text-muted hover:text-danger hover:border-danger transition"
               >