fix: use custom exceptions in onboarding and add tests

- Create onboarding-specific exceptions (OnboardingNotFoundException, etc.)
- Remove HTTPException usage from API endpoints per architecture rules
- Let exceptions propagate to global handler
- Add 12 integration tests for onboarding API endpoints

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/exceptions/__init__.py

@@ -252,6 +252,17 @@ from .vendor_theme import (
     VendorThemeNotFoundException,
 )
 
+# Onboarding exceptions
+from .onboarding import (
+    LetzshopConnectionFailedException,
+    OnboardingAlreadyCompletedException,
+    OnboardingCsvUrlRequiredException,
+    OnboardingNotFoundException,
+    OnboardingStepOrderException,
+    OnboardingSyncJobNotFoundException,
+    OnboardingSyncNotCompleteException,
+)
+
 __all__ = [
     # Base exceptions
     "WizamartException",
@@ -437,4 +448,12 @@ __all__ = [
     "SubscriptionAlreadyCancelledException",
     "InvalidWebhookSignatureException",
     "WebhookMissingSignatureException",
+    # Onboarding exceptions
+    "OnboardingNotFoundException",
+    "OnboardingStepOrderException",
+    "OnboardingAlreadyCompletedException",
+    "LetzshopConnectionFailedException",
+    "OnboardingCsvUrlRequiredException",
+    "OnboardingSyncJobNotFoundException",
+    "OnboardingSyncNotCompleteException",
 ]