feat: add CommonModule with PurgeService and global exception filter

Introduced CommonModule to centralize shared functionality. Added PurgeService for automated database cleanup and a global exception filter for unified error handling.

backend/src/common/interfaces/request.interface.ts

@@ -0,0 +1,8 @@
+import { Request } from "express";
+
+export interface AuthenticatedRequest extends Request {
+	user: {
+		sub: string;
+		username: string;
+	};
+}