feat(api): stats endpoint for gethomepage widget (closes #131)

Adds GET /api/v1/stats/summary, gated by HOMEPAGE_API_KEY env var and
X-API-Key header (hmac.compare_digest, matches the liveview pattern).

Payload:
  nodes / online / offline / unknown
  pending_devices (status='pending')
  zigbee_devices  (Node.ieee_address IS NOT NULL)
  last_scan_at    (max ScanRun.finished_at)

Disabled by default — endpoint returns 403 unless HOMEPAGE_API_KEY is set.
README documents activation and ships a ready-to-paste gethomepage
`customapi` widget snippet.

Tests cover: disabled-by-default, missing header, wrong key, empty DB,
and full aggregation across nodes/pending/zigbee/scan-runs.

backend/app/main.py

@@ -7,7 +7,7 @@ from typing import Any
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from app.api.routes import auth, canvas, edges, liveview, nodes, scan, status, zigbee
+from app.api.routes import auth, canvas, edges, liveview, nodes, scan, stats, status, zigbee
 from app.api.routes import settings as settings_routes
 from app.core.config import settings
 from app.core.scheduler import start_scheduler, stop_scheduler
@@ -56,6 +56,7 @@ app.include_router(status.router, prefix="/api/v1/status", tags=["status"])
 app.include_router(settings_routes.router, prefix="/api/v1/settings", tags=["settings"])
 app.include_router(liveview.router, prefix="/api/v1/liveview", tags=["liveview"])
 app.include_router(zigbee.router, prefix="/api/v1/zigbee", tags=["zigbee"])
+app.include_router(stats.router, prefix="/api/v1/stats", tags=["stats"])
 
 
 @app.get("/api/v1/health")