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/config.py

@@ -126,6 +126,11 @@ class Settings(BaseSettings):
     PASSWORD_RESET_TOKEN_EXPIRE_MINUTES: int = 30
     APP_BASE_URL: str = "http://localhost:3000"
 
+    # Signal trending and site alerting (Phase 15)
+    SIGNAL_DEGRADATION_THRESHOLD_DB: int = 5
+    ALERT_EVALUATION_INTERVAL_SECONDS: int = 300
+    TREND_DETECTION_INTERVAL_SECONDS: int = 3600
+
     # Retention cleanup — delete config snapshots older than N days
     CONFIG_RETENTION_DAYS: int = 90