Add FastAPI backend and React frontend subprojects

Backend:
- FastAPI app with 17 REST endpoints covering dashboard, monitoring,
  media index, file browser, and jobs
- Reuses existing clients/domain/services unchanged
- pydantic-settings config, dependency injection, CORS setup
- Auto-generated OpenAPI docs at /docs

Frontend:
- Vite + React + TypeScript SPA
- @tanstack/react-query for data fetching with polling
- ag-grid-react for media table and file browser
- recharts for monitoring charts
- Tailwind CSS styling
- 4 pages: Dashboard, Monitoring, Media, File Browser
- Typed API client matching all backend endpoints

Also:
- docs/MIGRATION_PLAN.md with full architecture plan
- Updated .gitignore for both subprojects
- Streamlit app preserved for now (can coexist)

frontend/src/hooks/useDashboard.ts

@@ -0,0 +1,26 @@
+import { useQuery } from "@tanstack/react-query";
+import { fetchCounts, fetchLibraries, fetchNowPlaying } from "../api/client";
+
+export function useCounts() {
+	return useQuery({
+		queryKey: ["dashboard", "counts"],
+		queryFn: fetchCounts,
+		staleTime: 5 * 60 * 1000,
+	});
+}
+
+export function useLibraries() {
+	return useQuery({
+		queryKey: ["dashboard", "libraries"],
+		queryFn: fetchLibraries,
+		staleTime: 5 * 60 * 1000,
+	});
+}
+
+export function useNowPlaying() {
+	return useQuery({
+		queryKey: ["dashboard", "now-playing"],
+		queryFn: fetchNowPlaying,
+		refetchInterval: 15_000,
+	});
+}