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/ChannelPage.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { useNavigationActions } from "./NavigationProvider";
 import {
   ArrowLeft,
@@ -73,7 +74,7 @@ export default function ChannelPage({
   const [bannerAttempt, setBannerAttempt] = useState(0);
 
   const detail = useQuery({
-    queryKey: ["channel", channelId],
+    queryKey: qk.channel(channelId),
     queryFn: () => api.channelDetail(channelId),
   });
   const ch = detail.data;
@@ -91,10 +92,10 @@ export default function ChannelPage({
         .exploreChannel(channelId, token ?? undefined)
         .then((r) => {
           setNextToken(r.next_page_token);
-          qc.invalidateQueries({ queryKey: ["feed"] });
+          qc.invalidateQueries({ queryKey: qk.feed() });
           // Refetch the detail so `explored` flips true → the "Exploring" badge shows on the
           // first visit (the GET that drove this render predated the explore that just ran).
-          qc.invalidateQueries({ queryKey: ["channel", channelId] });
+          qc.invalidateQueries({ queryKey: qk.channel(channelId) });
         })
         .finally(() => setExploring(false));
     },
@@ -116,9 +117,9 @@ export default function ChannelPage({
   const block = useMutation({
     mutationFn: () => (ch?.blocked ? api.unblockChannel(channelId) : api.blockChannel(channelId)),
     onSuccess: () => {
-      qc.invalidateQueries({ queryKey: ["channel", channelId] });
-      qc.invalidateQueries({ queryKey: ["feed"] });
-      qc.invalidateQueries({ queryKey: ["blocked-channels"] });
+      qc.invalidateQueries({ queryKey: qk.channel(channelId) });
+      qc.invalidateQueries({ queryKey: qk.feed() });
+      qc.invalidateQueries({ queryKey: qk.blockedChannels() });
     },
     // No onError: block/unblock is a local DB op (not a YouTube write, so no 403 "connect" case),
     // and its 400/409/422/500 are already surfaced by the global error modal (see api.ts req) —
@@ -128,12 +129,12 @@ export default function ChannelPage({
   const subscribe = useMutation({
     mutationFn: () => api.subscribeChannel(channelId),
     onSuccess: () => {
-      qc.invalidateQueries({ queryKey: ["channel", channelId] });
-      qc.invalidateQueries({ queryKey: ["feed"] });
-      qc.invalidateQueries({ queryKey: ["channels"] });
+      qc.invalidateQueries({ queryKey: qk.channel(channelId) });
+      qc.invalidateQueries({ queryKey: qk.feed() });
+      qc.invalidateQueries({ queryKey: qk.channels() });
       // Match ChannelDiscovery: the channel leaves discovery and the manager's stats move.
-      qc.invalidateQueries({ queryKey: ["my-status"] });
-      qc.invalidateQueries({ queryKey: ["discovered-channels"] });
+      qc.invalidateQueries({ queryKey: qk.myStatus() });
+      qc.invalidateQueries({ queryKey: qk.discoveredChannels() });
       notify({ level: "info", message: t("channel.subscribed", { name: ch?.title ?? "" }) });
     },
     // A 403 (missing write scope) otherwise fails silently on the channel page (no wizard handle
@@ -143,9 +144,9 @@ export default function ChannelPage({
   const unsubscribe = useMutation({
     mutationFn: () => api.unsubscribeChannel(channelId),
     onSuccess: () => {
-      qc.invalidateQueries({ queryKey: ["channel", channelId] });
-      qc.invalidateQueries({ queryKey: ["feed"] });
-      qc.invalidateQueries({ queryKey: ["channels"] });
+      qc.invalidateQueries({ queryKey: qk.channel(channelId) });
+      qc.invalidateQueries({ queryKey: qk.feed() });
+      qc.invalidateQueries({ queryKey: qk.channels() });
     },
     onError: (err) => notifyYouTubeActionError(err, t("channels.notify.unsubscribeFailed")),
   });