fix: use valid STARTER tier in onboarding tenant creation

OnboardingService passed "LOW" as the tier, but the Tier enum only has
STARTER/TEAM/BUSINESS/ENTERPRISE. Tier.valueOf("LOW") threw
IllegalArgumentException, which the controller caught as a blanket 409
Conflict — masking the real cause. Also catch IllegalStateException
(user already has a tenant) to return 409 instead of 500.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/java/net/siegeln/cameleer/saas/onboarding/OnboardingController.java

@@ -58,7 +58,9 @@ public class OnboardingController {
             TenantEntity tenant = onboardingService.createTrialTenant(request.name(), request.slug(), userId);
             return ResponseEntity.status(HttpStatus.CREATED).body(TenantResponse.from(tenant));
         } catch (IllegalArgumentException e) {
-            return ResponseEntity.status(HttpStatus.CONFLICT).build();
+            return ResponseEntity.status(HttpStatus.CONFLICT).body(null);
+        } catch (IllegalStateException e) {
+            return ResponseEntity.status(HttpStatus.CONFLICT).body(null);
         }
     }
 }

src/main/java/net/siegeln/cameleer/saas/onboarding/OnboardingService.java

@@ -41,7 +41,7 @@ public class OnboardingService {
 
         // Create tenant via the existing vendor flow (no admin user — we'll add the caller)
         UUID actorId = resolveActorId(logtoUserId);
-        var request = new CreateTenantRequest(name, slug, "LOW", null, null);
+        var request = new CreateTenantRequest(name, slug, "STARTER", null, null);
         TenantEntity tenant = vendorTenantService.createAndProvision(request, actorId);
 
         // Add the calling user to the Logto org as owner