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.

src/db/files.ts

@@ -10,7 +10,7 @@ export interface FileRow {
   created_at: string;
 }
 
-export interface CreateFileParams {
+interface CreateFileParams {
   id: string;
   userId: number;
   originalName: string;

src/db/users.ts

@@ -1,13 +1,13 @@
 import type Database from 'better-sqlite3';
 
-export interface UserRow {
+interface UserRow {
   id: number;
   username: string;
   password_hash: string;
   created_at: string;
 }
 
-export interface CreateUserParams {
+interface CreateUserParams {
   username: string;
   passwordHash: string;
 }

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;