feat: add mandatory vendor onboarding wizard

Implement 4-step onboarding flow for new vendors after signup:
- Step 1: Company profile setup
- Step 2: Letzshop API configuration with connection testing
- Step 3: Product & order import CSV URL configuration
- Step 4: Historical order sync with progress bar

Key features:
- Blocks dashboard access until completed
- Step indicators with visual progress
- Resume capability (progress persisted in DB)
- Admin skip capability for support cases

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

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

app/services/platform_signup_service.py

@@ -23,6 +23,7 @@ from app.exceptions import (
     ValidationException,
 )
 from app.services.email_service import EmailService
+from app.services.onboarding_service import OnboardingService
 from app.services.stripe_service import stripe_service
 from middleware.auth import AuthManager
 from models.database.company import Company
@@ -372,6 +373,10 @@ class PlatformSignupService:
         )
         db.add(vendor_user)
 
+        # Create VendorOnboarding record
+        onboarding_service = OnboardingService(db)
+        onboarding_service.create_onboarding(vendor.id)
+
         # Create Stripe Customer
         stripe_customer_id = stripe_service.create_customer(
             vendor=vendor,
@@ -615,11 +620,12 @@ class PlatformSignupService:
 
         logger.info(f"Completed signup for vendor {vendor_id}")
 
+        # Redirect to onboarding instead of dashboard
         return SignupCompletionResult(
             success=True,
             vendor_code=vendor_code,
             vendor_id=vendor_id,
-            redirect_url=f"/vendor/{vendor_code}/dashboard",
+            redirect_url=f"/vendor/{vendor_code}/onboarding",
             trial_ends_at=trial_ends_at.isoformat(),
         )