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/routers/media.py

@@ -0,0 +1,84 @@
+"""Media router — index status, build, and query."""
+
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import APIRouter, Depends, Query
+
+from dependencies import get_jellyfin_client, get_user_id
+from clients.jellyfin import JellyfinClient
+from services.media_index import MediaIndex, build_media_index
+
+router = APIRouter(prefix="/api/media", tags=["media"])
+
+
+def get_media_index() -> MediaIndex:
+    return MediaIndex()
+
+
+@router.get("/status")
+def get_index_status(index: MediaIndex = Depends(get_media_index)) -> dict[str, Any]:
+    """Return media index status (exists, count, last updated, build duration)."""
+    status = index.status()
+    return {
+        "exists": status.exists,
+        "item_count": status.item_count,
+        "updated_at": status.updated_at,
+        "updated_at_label": status.updated_at_label,
+        "build_duration_seconds": status.build_duration_seconds,
+    }
+
+
+@router.post("/build")
+def post_build_index(
+    client: JellyfinClient = Depends(get_jellyfin_client),
+    user_id: str = Depends(get_user_id),
+    index: MediaIndex = Depends(get_media_index),
+) -> dict[str, Any]:
+    """Rebuild the media index from Jellyfin."""
+    libraries = client.libraries(user_id)
+    count = build_media_index(client, user_id, libraries, index)
+    return {"indexed_items": count}
+
+
+@router.get("/query")
+def query_media(
+    libraries: str = Query("", description="Comma-separated library IDs"),
+    types: str = Query("Movie,Episode", description="Comma-separated media types"),
+    search: str = Query("", description="Search term"),
+    hdr_filter: str = Query("All", description="All, HDR only, SDR/unknown only"),
+    sort_key: str = Query("title", description="Sort field"),
+    sort_order: str = Query("Ascending", description="Ascending or Descending"),
+    limit: int = Query(100, ge=1, le=1000),
+    offset: int = Query(0, ge=0),
+    client: JellyfinClient = Depends(get_jellyfin_client),
+    user_id: str = Depends(get_user_id),
+    index: MediaIndex = Depends(get_media_index),
+) -> dict[str, Any]:
+    """Query the media index with filters, sorting, and pagination."""
+    # If no library IDs provided, use all libraries
+    library_ids = [lid.strip() for lid in libraries.split(",") if lid.strip()] if libraries else None
+    if not library_ids:
+        all_libs = client.libraries(user_id)
+        library_ids = [lib["Id"] for lib in all_libs]
+
+    media_types = [t.strip() for t in types.split(",") if t.strip()]
+
+    rows, total = index.query(
+        library_ids=library_ids,
+        media_types=media_types,
+        search=search,
+        hdr_filter=hdr_filter,
+        sort_key=sort_key,
+        sort_order=sort_order,
+        limit=limit,
+        offset=offset,
+    )
+
+    return {
+        "items": rows,
+        "total": total,
+        "limit": limit,
+        "offset": offset,
+    }