refactor(fe): migrate plex/messaging/admin/downloads/notifications to qk (R7 S2b)

Extend lib/queryKeys.ts to the remaining ~34 key roots and migrate every
remaining literal (queryKey: [...] AND the positional useLiveQuery keys) in
36 files. All ~200 React Query keys across the app now route through the
factory; zero raw key literals remain outside queryKeys.ts.

The heterogeneous composite plex keys (plex-library/facets/collections/
playlists carry a filters object, a ratingKeys array, or a union/group
discriminator) take a variadic pass-through — the factory centralizes the
ROOT string; single-id keys stay precisely typed. Nullable keys (thread,
playlist, ytSearch) accept null so a null segment is preserved.

Pure substitution — byte-identical key arrays, so TanStack prefix matching
is unchanged. Gate green: typecheck (app+node+e2e), eslint (0 err),
prettier, 68 vitest. Runtime-verified on a fresh dev server: Feed + Plex
modules load, feed/facets/tags/my-status/plex-library/plex-facets/
plex-collections/plex-playlists all 200, full UI renders.

frontend/src/components/AdminUsers.tsx

@@ -1,4 +1,5 @@
 import { useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Ban, Check, RotateCcw, Shield, Trash2, UserPlus, X } from "lucide-react";
@@ -70,7 +71,7 @@ function UsersRoles({ me }: { me: Me }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();
-  const q = useQuery({ queryKey: ["admin-users"], queryFn: api.adminUsers });
+  const q = useQuery({ queryKey: qk.adminUsers(), queryFn: api.adminUsers });
   // Which rows have an action in flight, so we disable ONLY those rows' buttons (SC1) — a Set, since
   // with per-row disabling the admin can now start actions on several rows concurrently.
   const [pendingIds, setPendingIds] = useState<Set<number>>(new Set());
@@ -86,7 +87,7 @@ function UsersRoles({ me }: { me: Me }) {
       api.setUserRole(id, role),
     onMutate: (vars) => startPending(vars.id),
     onSuccess: (_d, vars) => {
-      qc.invalidateQueries({ queryKey: ["admin-users"] });
+      qc.invalidateQueries({ queryKey: qk.adminUsers() });
       notify({ level: "success", message: t("users.roles.updated", { email: vars.email }) });
     },
     onSettled: (_d, _e, vars) => endPending(vars.id),
@@ -98,7 +99,7 @@ function UsersRoles({ me }: { me: Me }) {
       api.setUserSuspended(id, suspended),
     onMutate: (vars) => startPending(vars.id),
     onSuccess: (_d, vars) => {
-      qc.invalidateQueries({ queryKey: ["admin-users"] });
+      qc.invalidateQueries({ queryKey: qk.adminUsers() });
       notify({
         level: "success",
         message: t(vars.suspended ? "users.suspend.done" : "users.suspend.undone", {
@@ -113,7 +114,7 @@ function UsersRoles({ me }: { me: Me }) {
     mutationFn: ({ id }: { id: number; email: string }) => api.adminDeleteUser(id),
     onMutate: (vars) => startPending(vars.id),
     onSuccess: (_d, vars) => {
-      qc.invalidateQueries({ queryKey: ["admin-users"] });
+      qc.invalidateQueries({ queryKey: qk.adminUsers() });
       notify({ level: "success", message: t("users.delete.done", { email: vars.email }) });
     },
     onSettled: (_d, _e, vars) => endPending(vars.id),
@@ -267,11 +268,11 @@ function AdminInvites() {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();
-  const invites = useQuery({ queryKey: ["admin-invites"], queryFn: () => api.adminInvites() });
+  const invites = useQuery({ queryKey: qk.adminInvites(), queryFn: () => api.adminInvites() });
   const [newEmail, setNewEmail] = useState("");
   const refresh = () => {
-    qc.invalidateQueries({ queryKey: ["admin-invites"] });
-    qc.invalidateQueries({ queryKey: ["me"] }); // updates the pending badge
+    qc.invalidateQueries({ queryKey: qk.adminInvites() });
+    qc.invalidateQueries({ queryKey: qk.me() }); // updates the pending badge
   };
   const approve = useMutation({
     mutationFn: ({ id }: { id: number; email: string }) => api.approveInvite(id),
@@ -383,14 +384,14 @@ function AdminDemo() {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();
-  const list = useQuery({ queryKey: ["demo-whitelist"], queryFn: () => api.adminDemoWhitelist() });
+  const list = useQuery({ queryKey: qk.demoWhitelist(), queryFn: () => api.adminDemoWhitelist() });
   const [newEmail, setNewEmail] = useState("");
 
   const add = useMutation({
     mutationFn: (email: string) => api.addDemoWhitelist(email),
     onSuccess: (_d, email) => {
       setNewEmail("");
-      qc.invalidateQueries({ queryKey: ["demo-whitelist"] });
+      qc.invalidateQueries({ queryKey: qk.demoWhitelist() });
       notify({ level: "success", message: t("settings.demo.added", { email }) });
     },
     onError: () => notify({ level: "error", message: t("settings.demo.addFailed") }),
@@ -398,7 +399,7 @@ function AdminDemo() {
   const remove = useMutation({
     mutationFn: (id: number) => api.removeDemoWhitelist(id),
     onSuccess: () => {
-      qc.invalidateQueries({ queryKey: ["demo-whitelist"] });
+      qc.invalidateQueries({ queryKey: qk.demoWhitelist() });
       notify({ level: "success", message: t("settings.demo.removed") });
     },
     onError: () => notify({ level: "error", message: t("settings.demo.removeFailed") }),