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/common.module.ts

@@ -0,0 +1,11 @@
+import { Global, Module } from "@nestjs/common";
+import { DatabaseModule } from "../database/database.module";
+import { PurgeService } from "./services/purge.service";
+
+@Global()
+@Module({
+	imports: [DatabaseModule],
+	providers: [PurgeService],
+	exports: [PurgeService],
+})
+export class CommonModule {}