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/NowPlaying.tsx

@@ -0,0 +1,46 @@
+import type { NowPlayingSession } from "../types";
+
+interface Props {
+	sessions: NowPlayingSession[];
+}
+
+export function NowPlaying({ sessions }: Props) {
+	if (sessions.length === 0) {
+		return (
+			<p className="text-sm text-gray-500">
+				No active playback sessions right now.
+			</p>
+		);
+	}
+
+	return (
+		<div className="overflow-x-auto">
+			<table className="w-full text-sm border-collapse">
+				<thead>
+					<tr className="border-b text-left text-gray-600">
+						<th className="py-2 pr-4">User</th>
+						<th className="py-2 pr-4">Title</th>
+						<th className="py-2 pr-4">Type</th>
+						<th className="py-2 pr-4">State</th>
+						<th className="py-2 pr-4">Transcoding</th>
+						<th className="py-2 pr-4">Transcode type</th>
+						<th className="py-2 pr-4">Device</th>
+					</tr>
+				</thead>
+				<tbody>
+					{sessions.map((s) => (
+						<tr key={s.session_id} className="border-b hover:bg-gray-50">
+							<td className="py-2 pr-4 font-medium">{s.user}</td>
+							<td className="py-2 pr-4">{s.title}</td>
+							<td className="py-2 pr-4">{s.type}</td>
+							<td className="py-2 pr-4">{s.state}</td>
+							<td className="py-2 pr-4">{s.transcoding}</td>
+							<td className="py-2 pr-4">{s.transcoding_type}</td>
+							<td className="py-2 pr-4">{s.device}</td>
+						</tr>
+					))}
+				</tbody>
+			</table>
+		</div>
+	);
+}