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/types/index.ts

@@ -0,0 +1,126 @@
+/**
+ * Shared TypeScript interfaces matching backend API responses.
+ */
+
+export interface MediaCounts {
+	movies: number;
+	series: number;
+	episodes: number;
+}
+
+export interface LibraryCount {
+	library: string;
+	type: string;
+	movies: number;
+	series: number;
+	episodes: number;
+	total: number;
+}
+
+export interface NowPlayingSession {
+	user: string;
+	title: string;
+	type: string;
+	state: string;
+	transcoding: string;
+	transcoding_type: string;
+	device: string;
+	session_id: string;
+}
+
+export interface MonitoringStatus {
+	status: string;
+}
+
+export interface MonitoringSample {
+	ts: number;
+	cpu_pct: number;
+	iowait_pct?: number;
+	mem_pct: number;
+	net_rx_bytes_per_sec: number;
+	net_tx_bytes_per_sec: number;
+	disk_read_bps: number;
+	disk_write_bps: number;
+}
+
+export interface MonitoringMetrics {
+	samples: MonitoringSample[];
+	total_samples: number;
+	filtered_samples: number;
+	cutoff_ts: number;
+}
+
+export interface DiskSpace {
+	filesystem: string;
+	size: number;
+	used: number;
+	available: number;
+	used_pct: string;
+	mount: string;
+}
+
+export interface MediaIndexStatus {
+	exists: boolean;
+	item_count: number;
+	updated_at: number | null;
+	updated_at_label: string;
+	build_duration_seconds: number | null;
+}
+
+export interface MediaItem {
+	id: string;
+	title: string;
+	series: string;
+	season: string;
+	episode: number | null;
+	type: string;
+	year: number | null;
+	runtime_min: number | null;
+	size: string;
+	bitrate: string;
+	hdr: string;
+	video: string;
+	resolution: string;
+	date_added: string;
+	library: string;
+	path: string;
+}
+
+export interface MediaQueryResponse {
+	items: MediaItem[];
+	total: number;
+	limit: number;
+	offset: number;
+}
+
+export interface FileEntry {
+	type: string;
+	size: number;
+	mtime: number;
+	name: string;
+}
+
+export interface DirectoryListing {
+	path: string;
+	entries: FileEntry[];
+	count: number;
+}
+
+export interface JobTemplate {
+	key: string;
+	name: string;
+	description: string;
+}
+
+export interface JobResult {
+	job_key: string;
+	path: string;
+	exit_status: number;
+	stdout: string;
+	stderr: string;
+}
+
+export interface ResolvedPath {
+	original: string;
+	resolved: string;
+}