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/auth.ts

@@ -0,0 +1,46 @@
+/**
+ * Input for the login endpoint.
+ * The caller provides the admin API token to obtain a session cookie.
+ */
+export interface LoginInput {
+  /** Admin API token for authentication */
+  token: string;
+}
+
+/**
+ * Active authentication session information.
+ */
+export interface AuthSession {
+  /** Authenticated username (currently always "admin") */
+  username: string;
+  /** Session expiry timestamp; null for bearer-token sessions */
+  expiresAt: Date | null;
+  /** Authentication method used */
+  method: 'cookie' | 'bearer';
+}
+
+/**
+ * Response payload for a successful login.
+ */
+export interface LoginResponse {
+  /** Authenticated username */
+  username: string;
+}
+
+/**
+ * Response payload for logout.
+ */
+export interface LogoutResponse {
+  /** Whether the logout succeeded */
+  success: boolean;
+}
+
+/**
+ * Response payload for the current-user (/me) endpoint.
+ */
+export interface UserInfoResponse {
+  /** Authenticated username */
+  username: string;
+  /** ISO-8601 session expiry timestamp; null when using bearer token */
+  expiresAt: string | null;
+}