서버 코드를 Router, Service, Repository 계층으로 분리하고 오류 처리 및 파일 삭제 로직 개선

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/server/services/common.ts

@@ -0,0 +1,10 @@
+import { AppError } from "#lib/server/errors.js";
+
+const DEK_VERSION_TOLERANCE = 60 * 1000; // 1 minute
+
+// DEK versions are creation timestamps generated by clients
+export const ensureFreshDekVersion = (dekVersion: Date) => {
+  if (Math.abs(dekVersion.getTime() - Date.now()) >= DEK_VERSION_TOLERANCE) {
+    throw new AppError("BAD_REQUEST", "Invalid DEK version");
+  }
+};