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/ChannelDiscovery.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { UserPlus } from "lucide-react";
 import { api, type DiscoveredChannel } from "../lib/api";
 import { accountKey, LS, readAccount, writeAccount } from "../lib/storage";
@@ -52,7 +53,7 @@ export default function ChannelDiscovery({
   };
 
   const query = useQuery({
-    queryKey: ["discovered-channels"],
+    queryKey: qk.discoveredChannels(),
     queryFn: api.discoveredChannels,
   });
 
@@ -61,9 +62,9 @@ export default function ChannelDiscovery({
     onSuccess: (_data, c) => {
       // The channel moves from "discovery" to "subscribed", and its videos will start
       // arriving — refresh both lists plus the per-user status.
-      qc.invalidateQueries({ queryKey: ["discovered-channels"] });
-      qc.invalidateQueries({ queryKey: ["channels"] });
-      qc.invalidateQueries({ queryKey: ["my-status"] });
+      qc.invalidateQueries({ queryKey: qk.discoveredChannels() });
+      qc.invalidateQueries({ queryKey: qk.channels() });
+      qc.invalidateQueries({ queryKey: qk.myStatus() });
       const name = c.title ?? c.id;
       // Name the channel and ship a typed payload so the inbox can offer "open in the
       // Channel manager" / "open on YouTube" links — kept after reload, when live