feat(services): select Jellyfin via jellyfin_service_id on the frontend

Slice 4b frontend half. Jellyfin-touching pages now select a Jellyfin service
instance instead of a machine.

- api/client.ts: Jellyfin-backed calls (counts/libraries/activity/users, media
  status/build/stop/force-stop, queryMedia) send jellyfin_service_id.
- hooks/useDashboard, useUsers, useMedia: selector param renamed to
  jellyfinServiceId.
- pages/Media + Applications: list jellyfin service instances and persist
  jellyfin_service_id in the URL.
- Dashboard (widgets) and Users (default instance) need no selector change.
- Update Applications + Media tests for the new hook/param.

Files/SSH transport keeps machine_id. Verification: frontend lint 0 errors,
build success, 70 tests; backend ruff clean, 222 tests.

frontend/src/hooks/useMedia.ts

@@ -7,10 +7,10 @@ import {
 	forceStopMediaIndexBuild,
 } from "../api/client";
 
-export function useMediaStatus(machineId?: string) {
+export function useMediaStatus(jellyfinServiceId?: string) {
 	return useQuery({
-		queryKey: ["media", "status", machineId ?? "default"],
-		queryFn: () => fetchMediaStatus(machineId),
+		queryKey: ["media", "status", jellyfinServiceId ?? "default"],
+		queryFn: () => fetchMediaStatus(jellyfinServiceId),
 		staleTime: 5_000,
 		refetchInterval: (query) =>
 			query.state.data?.build_running ? 1000 : false,
@@ -27,7 +27,7 @@ export function useMediaQuery(params: {
 	sort_order?: string;
 	limit?: number;
 	offset?: number;
-	machineId?: string;
+	jellyfinServiceId?: string;
 	enabled?: boolean;
 }) {
 	const { enabled = true, ...queryParams } = params;
@@ -44,30 +44,30 @@ function invalidateMedia(queryClient: ReturnType<typeof useQueryClient>) {
 	queryClient.invalidateQueries({ queryKey: ["media"] });
 }
 
-export function useBuildIndex(machineId?: string) {
+export function useBuildIndex(jellyfinServiceId?: string) {
 	const queryClient = useQueryClient();
 	return useMutation({
-		mutationFn: () => buildMediaIndex(machineId),
+		mutationFn: () => buildMediaIndex(jellyfinServiceId),
 		onSuccess: () => {
 			invalidateMedia(queryClient);
 		},
 	});
 }
 
-export function useStopBuildIndex(machineId?: string) {
+export function useStopBuildIndex(jellyfinServiceId?: string) {
 	const queryClient = useQueryClient();
 	return useMutation({
-		mutationFn: () => stopMediaIndexBuild(machineId),
+		mutationFn: () => stopMediaIndexBuild(jellyfinServiceId),
 		onSuccess: () => {
 			invalidateMedia(queryClient);
 		},
 	});
 }
 
-export function useForceStopBuildIndex(machineId?: string) {
+export function useForceStopBuildIndex(jellyfinServiceId?: string) {
 	const queryClient = useQueryClient();
 	return useMutation({
-		mutationFn: () => forceStopMediaIndexBuild(machineId),
+		mutationFn: () => forceStopMediaIndexBuild(jellyfinServiceId),
 		onSuccess: () => {
 			invalidateMedia(queryClient);
 		},