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/core/config.py

@@ -61,6 +61,11 @@ class Settings(BaseSettings):
     # Leave unset (or empty) to keep the feature disabled (default).
     liveview_key: str | None = None
 
+    # Homepage widget — optional read-only stats endpoint for gethomepage.
+    # Set to a random secret to enable /api/v1/stats/summary (X-API-Key header).
+    # Leave empty to keep the feature disabled (default).
+    homepage_api_key: str = ""
+
     def _override_path(self) -> Path:
         return Path(self.sqlite_path).parent / "scan_config.json"