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.

.env.example

@@ -28,3 +28,9 @@ MCP_SERVICE_KEY=svc_changeme
 # Off by default. Set to a random secret to enable.
 # Generate: python3 -c "import secrets; print(secrets.token_urlsafe(32))"
 # LIVEVIEW_KEY=
+
+# Gethomepage widget — read-only stats at /api/v1/stats/summary
+# Off by default. Set to a random secret to enable; clients must send
+# the same value in the `X-API-Key` header.
+# Generate: python3 -c "import secrets; print(secrets.token_urlsafe(32))"
+# HOMEPAGE_API_KEY=