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/useMedia.ts

@@ -0,0 +1,40 @@
+import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
+import { fetchMediaStatus, buildMediaIndex, queryMedia } from "../api/client";
+
+export function useMediaStatus() {
+	return useQuery({
+		queryKey: ["media", "status"],
+		queryFn: fetchMediaStatus,
+		staleTime: 60_000,
+	});
+}
+
+export function useMediaQuery(params: {
+	libraries?: string;
+	types?: string;
+	search?: string;
+	hdr_filter?: string;
+	sort_key?: string;
+	sort_order?: string;
+	limit?: number;
+	offset?: number;
+	enabled?: boolean;
+}) {
+	const { enabled = true, ...queryParams } = params;
+	return useQuery({
+		queryKey: ["media", "query", queryParams],
+		queryFn: () => queryMedia(queryParams),
+		enabled,
+		staleTime: 30_000,
+	});
+}
+
+export function useBuildIndex() {
+	const queryClient = useQueryClient();
+	return useMutation({
+		mutationFn: buildMediaIndex,
+		onSuccess: () => {
+			queryClient.invalidateQueries({ queryKey: ["media"] });
+		},
+	});
+}