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/components/MetricCard.tsx

@@ -0,0 +1,19 @@
+interface Props {
+	label: string;
+	value: string;
+	subtext?: string;
+}
+
+export function MetricCard({ label, value, subtext }: Props) {
+	return (
+		<div className="rounded-lg border p-4">
+			<p className="text-xs text-gray-500 uppercase tracking-wide">{label}</p>
+			<p className="text-2xl font-bold mt-1">{value}</p>
+			{subtext && (
+				<p className="text-xs text-gray-400 mt-1 whitespace-pre-line">
+					{subtext}
+				</p>
+			)}
+		</div>
+	);
+}