fix(file-service): permit /error endpoint to avoid 401 on server errors

Spring Boot's /error handler was secured, causing any server-side exception
during upload/serve to redirect clients to login instead of returning an
error message. Permitting /error ensures errors are returned as proper
JSON responses rather than auth challenges.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

file-service/src/main/java/com/xuqm/file/config/SecurityConfig.java

@@ -40,6 +40,7 @@ public class SecurityConfig {
                 .requestMatchers(new AntPathRequestMatcher("/api/file/*/thumbnail", "GET")).permitAll()
                 .requestMatchers(new AntPathRequestMatcher("/api/file/*", "GET")).permitAll()
                 .requestMatchers(new AntPathRequestMatcher("/actuator/**", "GET")).permitAll()
+                .requestMatchers(new AntPathRequestMatcher("/error")).permitAll()
                 .anyRequest().authenticated()
             )
             .exceptionHandling(ex -> ex