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/vendor.py

@@ -211,6 +211,14 @@ class Vendor(Base, TimestampMixin):
         "ContentPage", back_populates="vendor", cascade="all, delete-orphan"
     )  # Relationship with ContentPage model for vendor-specific content pages
 
+    # Onboarding progress (one-to-one)
+    onboarding = relationship(
+        "VendorOnboarding",
+        back_populates="vendor",
+        uselist=False,
+        cascade="all, delete-orphan",
+    )
+
     def __repr__(self):
         """String representation of the Vendor object."""
         return f"<Vendor(id={self.id}, vendor_code='{self.vendor_code}', name='{self.name}', subdomain='{self.subdomain}')>"