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/api/v1/vendor/__init__.py

@@ -28,6 +28,7 @@ from . import (
     media,
     messages,
     notifications,
+    onboarding,
     order_item_exceptions,
     orders,
     payments,
@@ -56,6 +57,7 @@ router.include_router(auth.router, tags=["vendor-auth"])
 router.include_router(dashboard.router, tags=["vendor-dashboard"])
 router.include_router(profile.router, tags=["vendor-profile"])
 router.include_router(settings.router, tags=["vendor-settings"])
+router.include_router(onboarding.router, tags=["vendor-onboarding"])
 
 # Business operations (with prefixes: /products/*, /orders/*, etc.)
 router.include_router(products.router, tags=["vendor-products"])