fix(notification-routing): harden with security fixes, validation, and test coverage (#573)

- Add authMiddleware to GET/POST /api/agents (previously unauthenticated)
- Add input validation to POST /api/agents (type, URL, enabled)
- Add URL host validation via new URL() to notification-routes POST/PUT
- Add priority type validation and enabled boolean check to routes
- Add name length limit (100 chars) to routes POST/PUT
- Add stack pattern dedup and whitespace filtering
- Add NaN guard to DELETE /api/notifications/:id
- Add dispatch_error to NotificationHistory TypeScript interface
- Extract cleanStackPatterns() and validateHttpsUrl() helpers
- Add standard and diagnostic logging to agents and routes endpoints
- Add comprehensive integration tests for notification-routes CRUD
- Add agents auth and validation tests
- Add dispatch error recording unit tests

backend/src/services/DatabaseService.ts

@@ -109,6 +109,7 @@ export interface NotificationHistory {
     message: string;
     timestamp: number;
     is_read: boolean;
+    dispatch_error?: string;
 }
 
 export interface FleetSnapshot {