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)

README.md

@@ -13,10 +13,8 @@ Project policy/docs:
 
 - Dashboard media counts for movies, series, and episodes, plus now-playing sessions (user, title, playback state, transcoding)
 - SQLite-indexed Media tab with full-library sort/filter for runtime, size, bitrate, explicit HDR yes/no flag, date added, codec, resolution, series, season, episode, path, and row-based selection that automatically syncs File browser to the selected item's folder
-- SSH resource overview dashboard plus detailed Resources tab for CPU, RAM, network, disk I/O, and disk space
+- Server monitoring dashboard plus detailed Monitoring tab for CPU, IO wait, RAM, network, disk I/O, and disk space
 - Jellyfin API-key connection using `GET /Users` plus user-scoped library endpoints
-- Library selection, search, pagination, poster grid
-- Item details with Jellyfin metadata and raw JSON
 - Compact SSH remote directory browser with clickable rows, search, filters, sorting, pagination, and `[UP] ..` navigation
 - Blocking selected-file `ffprobe` preview for known video files
 - Separate container, video, audio, and subtitle metadata sections