fix: correct malformed @Value annotations in DataInitializer

Missing closing braces caused Spring to inject the literal placeholder
string instead of resolving the property, silently ignoring any
app.admin.username / app.admin.password env-var overrides.

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

backend/src/main/java/org/raddatz/familienarchiv/config/DataInitializer.java

@@ -30,10 +30,10 @@ import java.util.concurrent.ThreadLocalRandom;
 @Slf4j
 public class DataInitializer {
 
-    @Value("${app.admin.username:admin")
+    @Value("${app.admin.username:admin}")
     private String adminUsername;
 
-    @Value("${app.admin.password:admin123")
+    @Value("${app.admin.password:admin123}")
     private String adminPassword;
 
     private final AppUserRepository userRepository;