feat: create application DTOs

Add data-transfer-object interfaces for the application layer:
upload, file, bucket, S3, and auth domains.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/application/dto/bucket.ts

@@ -0,0 +1,47 @@
+/**
+ * Input for creating a new bucket.
+ */
+export interface CreateBucketInput {
+  /** Bucket name (must match S3 naming rules: 3-63 chars, lowercase, no underscore) */
+  name: string;
+}
+
+/**
+ * Single bucket representation returned by bucket endpoints.
+ */
+export interface BucketResponse {
+  /** Bucket UUID */
+  id: string;
+  /** Bucket name */
+  name: string;
+  /** ISO-8601 timestamp of when the bucket was created */
+  createdAt: string;
+  /** Number of non-deleted objects in the bucket */
+  objectCount?: number;
+}
+
+/**
+ * Response payload for the list-buckets endpoint.
+ */
+export interface BucketListResponse {
+  /** Array of buckets */
+  buckets: BucketResponse[];
+}
+
+/**
+ * Response payload for bucket creation.
+ */
+export interface CreateBucketResponse {
+  /** Bucket UUID */
+  id: string;
+  /** Bucket name */
+  name: string;
+}
+
+/**
+ * Response payload for bucket deletion.
+ */
+export interface DeleteBucketResponse {
+  /** Whether the deletion succeeded */
+  success: boolean;
+}