refactor: simplify setup flow — eliminate redundant DB calls and login round-trips

- Add AdminExists() to UserRepository (EXISTS query, not full table scan)
- SetupInitialAdmin returns tokens directly (skip separate Login call)
- Add SetupRequest DTO to auth_dto.go (replace inline struct)
- Extract defaultEmail() helper (removes duplicated email logic)
- AuthPage uses setup tokens directly (skip redundant apiLogin call)
- Use customAxiosInstance for auth API calls (consistent with codebase)

frontend/src/features/auth/pages/AuthPage.tsx

@@ -59,20 +59,20 @@ const AuthPage: React.FC<Props> = ({ onLogin }) => {
     toastInfo("Creating admin account...", { title: "Setup", durationMs: 1200 });
 
     try {
-      await setupInitialAdmin({
+      const result = await setupInitialAdmin({
         username: setupUsername,
         password: setupPassword,
         email: setupEmail || undefined,
       });
 
-      // Login with the returned tokens
-      const loginResponse = await apiLogin({
+      // setupInitialAdmin returns tokens — use them directly to avoid redundant login
+      onLogin({
+        accessToken: result.accessToken,
+        refreshToken: result.refreshToken,
         username: setupUsername,
-        password: setupPassword,
-      });
+      } as any);
 
       toastSuccess("Admin account created. Welcome!");
-      onLogin(loginResponse);
       navigate("/home", { replace: true });
     } catch (err: unknown) {
       const msg = getErrorMessage(err, "Setup failed. Please try again later.");