feat(15-01): add site alert rules/events migration, models, schemas, and config

- Create Alembic migration 035 with site_alert_rules and site_alert_events tables, RLS policies, and GRANT
- Add SiteAlertRule/SiteAlertEvent ORM models with enums for rule_type, severity, state
- Add Pydantic schemas for rule/event CRUD and signal history points
- Add SIGNAL_DEGRADATION_THRESHOLD_DB, ALERT_EVALUATION_INTERVAL_SECONDS, TREND_DETECTION_INTERVAL_SECONDS to Settings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -21,6 +21,7 @@ from app.models.api_key import ApiKey
 from app.models.config_backup import RouterConfigSnapshot, RouterConfigDiff, RouterConfigChange
 from app.models.device_interface import DeviceInterface
 from app.models.wireless_link import WirelessLink, LinkState
+from app.models.site_alert import SiteAlertRule, SiteAlertEvent
 
 __all__ = [
     "Tenant",
@@ -52,4 +53,6 @@ __all__ = [
     "DeviceInterface",
     "WirelessLink",
     "LinkState",
+    "SiteAlertRule",
+    "SiteAlertEvent",
 ]