fix(user): fail-closed when admin seed would use dev defaults outside dev/test/e2e

Addresses Nora's review concern on #513/#516.

The previous fix only made env-vars take effect — it did NOT close the
fail-open default path. If an operator forgets APP_ADMIN_USERNAME /
APP_ADMIN_PASSWORD on first prod boot, the seeded admin is the
well-known `admin@familienarchiv.local` / `admin123` and is permanently
locked (UserDataInitializer only seeds when the row is missing).

Refuse to seed outside dev/test/e2e profiles when either credential
matches the documented default. The startup fails fast with a clear
message pointing at the env-var names and the permanence trap.

Also adds Markus/Felix/Sara's "pin the Java side" coverage: a
reflection test on the @Value placeholder catches a future rename
of `${app.admin.email:...}` back to `${app.admin.username:...}`,
which would otherwise pass the yaml-side test but silently break
the binding.

Tests:
- AdminSeedFailClosedTest pins fail-closed for non-local profiles
  and verifies the dev/test/e2e bypass.
- AdminSeedPropertyKeyTest now also asserts the @Value placeholder
  string on UserDataInitializer.adminEmail/adminPassword.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/user/UserDataInitializer.java

@@ -20,6 +20,7 @@ import org.springframework.boot.CommandLineRunner;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Profile;
+import org.springframework.core.env.Environment;
 import org.springframework.security.crypto.password.PasswordEncoder;
 
 import java.time.LocalDate;
@@ -31,19 +32,39 @@ import java.util.Set;
 @DependsOn("flyway")
 public class UserDataInitializer {
 
-    @Value("${app.admin.email:admin@familyarchive.local}")
+    static final String DEFAULT_ADMIN_EMAIL = "admin@familienarchiv.local";
+    static final String DEFAULT_ADMIN_PASSWORD = "admin123";
+
+    @Value("${app.admin.email:" + DEFAULT_ADMIN_EMAIL + "}")
     private String adminEmail;
 
-    @Value("${app.admin.password:admin123}")
+    @Value("${app.admin.password:" + DEFAULT_ADMIN_PASSWORD + "}")
     private String adminPassword;
 
     private final AppUserRepository userRepository;
     private final UserGroupRepository groupRepository;
+    private final Environment environment;
 
     @Bean
     public CommandLineRunner initAdminUser(PasswordEncoder passwordEncoder) {
         return args -> {
             if (userRepository.findByEmail(adminEmail).isEmpty()) {
+                // Fail-closed in production: refuse to seed with the well-known
+                // defaults. Otherwise an operator who forgets APP_ADMIN_USERNAME
+                // / APP_ADMIN_PASSWORD locks production to admin@…/admin123 PERMANENTLY
+                // (UserDataInitializer only seeds when the row is missing — see #513).
+                // Allowed in dev/test/e2e because those run without secrets configured.
+                boolean isLocalProfile = environment.matchesProfiles("dev", "test", "e2e");
+                if (!isLocalProfile
+                    && (DEFAULT_ADMIN_EMAIL.equals(adminEmail)
+                        || DEFAULT_ADMIN_PASSWORD.equals(adminPassword))) {
+                    throw new IllegalStateException(
+                        "Refusing to seed admin user with default credentials outside "
+                        + "the dev/test/e2e profiles. Set APP_ADMIN_USERNAME and "
+                        + "APP_ADMIN_PASSWORD to non-default values before first boot — "
+                        + "this lock-in is permanent."
+                    );
+                }
                 log.info("Kein Admin-User '{}' gefunden. Erstelle Default-Admin...", adminEmail);
 
                 UserGroup adminGroup = UserGroup.builder()