feat(backend): add customUpCodes support for HTTP monitors

Adds a customUpCodes field to the monitor schema to allow users to specify which HTTP status codes should be treated as UP instead of DOWN. Includes core logic in HttpProvider and unit tests.

Refs #3657

server/src/types/monitor.ts

@@ -60,6 +60,7 @@ export interface Monitor {
 	uptimePercentage?: number;
 	notifications: string[];
 	tags: string[];
+	customUpCodes: number[];
 	secret?: string;
 	cpuAlertThreshold: number;
 	cpuAlertCounter: number;