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/README.md

@@ -0,0 +1,61 @@
+# Frontend - Media Library Viewer
+
+React + TypeScript SPA for the Media Library Viewer, consuming the FastAPI backend.
+
+## Tech Stack
+
+- **Vite** — Build tool
+- **React 18+** — UI framework
+- **TypeScript** — Type safety
+- **@tanstack/react-query** — Data fetching/caching
+- **ag-grid-react** — Data tables (media, file browser)
+- **recharts** — Monitoring charts
+- **react-router-dom** — Client-side routing
+- **Tailwind CSS** — Styling
+
+## Setup
+
+```bash
+cd frontend
+npm install
+```
+
+## Development
+
+```bash
+npm run dev
+```
+
+Runs on http://localhost:5173 with API requests proxied to http://localhost:8000.
+
+Make sure the backend is running:
+
+```bash
+cd ../backend
+uvicorn main:app --reload --port 8000
+```
+
+## Build
+
+```bash
+npm run build
+```
+
+Output goes to `frontend/dist/`.
+
+## Pages
+
+- **Dashboard** (`/`) — Now playing, server overview, library stats
+- **Monitoring** (`/monitoring`) — CPU/IO wait/RAM/network/disk charts, collector controls
+- **Media** (`/media`) — Full-library table with sort/filter/search
+- **File Browser** (`/files`) — Remote directory browsing, ffprobe preview, jobs
+
+## Environment Variables
+
+Create a `.env` file in `frontend/` if the API is not at `http://localhost:8000`:
+
+```bash
+VITE_API_URL=http://your-backend-host:8000
+```
+
+In development, the Vite proxy handles `/api` requests automatically.