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>

models/database/__init__.py

@@ -47,6 +47,7 @@ from .marketplace_product import (
     ProductType,
 )
 from .marketplace_product_translation import MarketplaceProductTranslation
+from .onboarding import OnboardingStatus, OnboardingStep, VendorOnboarding
 from .order import Order, OrderItem
 from .order_item_exception import OrderItemException
 from .product import Product
@@ -153,4 +154,8 @@ __all__ = [
     "Message",
     "MessageAttachment",
     "ParticipantType",
+    # Onboarding
+    "OnboardingStatus",
+    "OnboardingStep",
+    "VendorOnboarding",
 ]