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/schema/__init__.py

@@ -10,6 +10,7 @@ from . import (
     marketplace_import_job,
     marketplace_product,
     message,
+    onboarding,
     stats,
     vendor,
 )
@@ -24,6 +25,7 @@ __all__ = [
     "marketplace_product",
     "message",
     "inventory",
+    "onboarding",
     "vendor",
     "marketplace_import_job",
     "stats",