refactor: remove unused exported type interfaces

CreateFileParams, UserRow, CreateUserParams, and MultipartFile were
exported but never imported outside their own modules. Narrowed visibility
to module-local to keep the public surface minimal.

Confirmed with knip (zero findings) and all 61 tests passing.

tests/helpers/setup.ts

@@ -18,7 +18,7 @@ export async function loginAs(app: FastifyInstance, username: string, password:
   return cookie.split(';')[0].replace('token=', '');
 }
 
-export interface MultipartFile {
+interface MultipartFile {
   filename: string;
   contentType: string;
   data: Buffer;