refactor(audit): extract createUserForBootstrap() to make null actorId contract explicit

createUserOrUpdate(UUID actorId, ...) is always called from the controller with
a real authenticated actor. createUserForBootstrap() handles seeding/test setup
without emitting an audit event, making the two contracts unambiguous.

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

backend/src/test/java/org/raddatz/familienarchiv/service/UserServiceTest.java

@@ -837,6 +837,26 @@ class UserServiceTest {
         verify(auditService, never()).logAfterCommit(any(), any(), any(), any());
     }
 
+    // ─── createUserForBootstrap ───────────────────────────────────────────────
+
+    @Test
+    void createUserForBootstrap_createsUserWithoutAuditEvent() {
+        CreateUserRequest req = new CreateUserRequest();
+        req.setEmail("bootstrap@example.com");
+        req.setInitialPassword("secret");
+        req.setGroupIds(List.of());
+
+        when(userRepository.findByEmail("bootstrap@example.com")).thenReturn(Optional.empty());
+        when(passwordEncoder.encode("secret")).thenReturn("encoded");
+        AppUser saved = AppUser.builder().id(UUID.randomUUID()).email("bootstrap@example.com").build();
+        when(userRepository.save(any())).thenReturn(saved);
+
+        AppUser result = userService.createUserForBootstrap(req);
+
+        assertThat(result).isEqualTo(saved);
+        verify(auditService, never()).logAfterCommit(any(), any(), any(), any());
+    }
+
     // ─── createGroup ──────────────────────────────────────────────────────────
 
     @Test