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/main.py

@@ -0,0 +1,51 @@
+"""FastAPI application entrypoint."""
+
+from __future__ import annotations
+
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from routers import dashboard, monitoring, media, files, jobs
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """Application lifespan — startup/shutdown."""
+    yield
+
+
+app = FastAPI(
+    title="Media Library Viewer API",
+    version="0.1.0",
+    description="Backend API for Jellyfin media browsing, SSH file inspection, and server monitoring.",
+    lifespan=lifespan,
+)
+
+# CORS for development (Vite runs on :5173)
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=[
+        "http://localhost:5173",
+        "http://localhost:3000",
+        "http://127.0.0.1:5173",
+        "http://127.0.0.1:3000",
+    ],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+# Register routers
+app.include_router(dashboard.router)
+app.include_router(monitoring.router)
+app.include_router(media.router)
+app.include_router(files.router)
+app.include_router(jobs.router)
+
+
+@app.get("/api/health")
+def health_check() -> dict[str, str]:
+    """Simple health check endpoint."""
+    return {"status": "ok"}