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/useFiles.ts

@@ -0,0 +1,49 @@
+import { useQuery, useMutation } from "@tanstack/react-query";
+import {
+	fetchDirectoryListing,
+	fetchFfprobe,
+	fetchStat,
+	fetchJobTemplates,
+	runJob,
+} from "../api/client";
+
+export function useDirectoryListing(path: string) {
+	return useQuery({
+		queryKey: ["files", "list", path],
+		queryFn: () => fetchDirectoryListing(path),
+		enabled: !!path,
+		staleTime: 30_000,
+	});
+}
+
+export function useFfprobe(path: string, enabled = false) {
+	return useQuery({
+		queryKey: ["files", "ffprobe", path],
+		queryFn: () => fetchFfprobe(path),
+		enabled: enabled && !!path,
+		staleTime: 5 * 60_000,
+	});
+}
+
+export function useStat(path: string, enabled = false) {
+	return useQuery({
+		queryKey: ["files", "stat", path],
+		queryFn: () => fetchStat(path),
+		enabled: enabled && !!path,
+	});
+}
+
+export function useJobTemplates() {
+	return useQuery({
+		queryKey: ["jobs", "templates"],
+		queryFn: fetchJobTemplates,
+		staleTime: 60_000,
+	});
+}
+
+export function useRunJob() {
+	return useMutation({
+		mutationFn: ({ jobKey, path }: { jobKey: string; path: string }) =>
+			runJob(jobKey, path),
+	});
+}