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/monitoring.py

@@ -0,0 +1,82 @@
+"""Monitoring router — metrics, collector controls, disk space."""
+
+from __future__ import annotations
+
+import time
+from typing import Any
+
+from fastapi import APIRouter, Depends
+
+from dependencies import get_ssh_client
+from clients.ssh import RemoteSSHClient
+from clients.resources import (
+    disk_space,
+    read_resource_metrics,
+    resource_collector_debug_info,
+    resource_collector_status,
+    restart_resource_collector,
+    start_resource_collector,
+    stop_resource_collector,
+)
+from config import get_settings
+
+router = APIRouter(prefix="/api/monitoring", tags=["monitoring"])
+
+
+@router.get("/status")
+def get_status(ssh: RemoteSSHClient = Depends(get_ssh_client)) -> dict[str, str]:
+    """Return collector running status."""
+    return {"status": resource_collector_status(ssh)}
+
+
+@router.get("/metrics")
+def get_metrics(
+    max_lines: int = 1000,
+    last_seconds: int = 3600,
+    ssh: RemoteSSHClient = Depends(get_ssh_client),
+) -> dict[str, Any]:
+    """Return resource metric samples from the remote collector."""
+    rows = read_resource_metrics(ssh, max_lines=max_lines)
+    cutoff_ts = time.time() - last_seconds
+    filtered = [row for row in rows if float(row.get("ts", 0)) >= cutoff_ts]
+    return {
+        "samples": filtered,
+        "total_samples": len(rows),
+        "filtered_samples": len(filtered),
+        "cutoff_ts": cutoff_ts,
+    }
+
+
+@router.get("/disk")
+def get_disk_space(ssh: RemoteSSHClient = Depends(get_ssh_client)) -> dict[str, Any]:
+    """Return disk space for the configured media root."""
+    settings = get_settings()
+    path = settings.media_root or "/"
+    return disk_space(ssh, path)
+
+
+@router.post("/start")
+def post_start(ssh: RemoteSSHClient = Depends(get_ssh_client)) -> dict[str, str]:
+    """Start the remote resource collector."""
+    message = start_resource_collector(ssh)
+    return {"message": message}
+
+
+@router.post("/stop")
+def post_stop(ssh: RemoteSSHClient = Depends(get_ssh_client)) -> dict[str, str]:
+    """Stop the remote resource collector."""
+    message = stop_resource_collector(ssh)
+    return {"message": message}
+
+
+@router.post("/restart")
+def post_restart(ssh: RemoteSSHClient = Depends(get_ssh_client)) -> dict[str, str]:
+    """Restart the remote resource collector."""
+    message = restart_resource_collector(ssh)
+    return {"message": message}
+
+
+@router.get("/diagnostics")
+def get_diagnostics(ssh: RemoteSSHClient = Depends(get_ssh_client)) -> dict[str, str]:
+    """Return collector debug info for troubleshooting."""
+    return {"diagnostics": resource_collector_debug_info(ssh)}