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)

backend/README.md

@@ -0,0 +1,65 @@
+# Backend - Media Library Viewer API
+
+FastAPI backend serving the REST API for Jellyfin media browsing, SSH file inspection, and server monitoring.
+
+## Setup
+
+```bash
+cd backend
+python -m venv .venv
+source .venv/bin/activate
+pip install -e '.[dev]'
+```
+
+Or install dependencies directly:
+
+```bash
+pip install fastapi uvicorn[standard] pydantic-settings paramiko requests python-dotenv pandas
+```
+
+## Configuration
+
+Create a `.env` file in the project root (or set environment variables):
+
+```bash
+JELLYFIN_URL=https://jellyfin.example.com
+JELLYFIN_API_KEY=your-api-key
+JELLYFIN_USER_ID=
+
+SSH_HOST=media-server.example.com
+SSH_USERNAME=username
+SSH_PORT=22
+SSH_KEY_FILENAME=/home/username/.ssh/id_rsa
+SSH_PASSWORD=
+
+REMOTE_MEDIA_ROOT=/srv/media
+REMOTE_PATH_PREFIX=
+```
+
+## Running
+
+```bash
+cd backend
+uvicorn main:app --reload --port 8000
+```
+
+API docs available at: http://localhost:8000/docs
+
+## API Endpoints
+
+- `GET /api/dashboard/counts` — Movie/series/episode totals
+- `GET /api/dashboard/libraries` — Per-library breakdown
+- `GET /api/dashboard/now-playing` — Active playback sessions
+- `GET /api/monitoring/status` — Collector status
+- `GET /api/monitoring/metrics` — Resource samples
+- `GET /api/monitoring/disk` — Disk space
+- `POST /api/monitoring/start|stop|restart` — Collector controls
+- `GET /api/media/status` — Index status
+- `POST /api/media/build` — Rebuild index
+- `GET /api/media/query` — Query with filters/sort/pagination
+- `GET /api/files/list?path=` — Directory listing
+- `GET /api/files/ffprobe?path=` — ffprobe JSON
+- `GET /api/files/stat?path=` — stat output
+- `GET /api/files/resolve-path?path=` — Path resolution
+- `GET /api/jobs/templates` — Available jobs
+- `POST /api/jobs/run` — Execute a job