Add simple auth

backend/src/main/java/com/aiteacher/config/SecurityConfig.java

@@ -30,9 +30,10 @@ public class SecurityConfig {
 
     @Bean
     public UserDetailsService userDetailsService(
+            @Value("${app.auth.username}") String username,
             @Value("${app.auth.password}") String password) {
         UserDetails user = User.builder()
-            .username("neurosurgeon")
+            .username(username)
             .password("{noop}" + password)
             .roles("USER")
             .build();