feat: add invoicing system and subscription tier enforcement

Phase 1 OMS implementation:

Invoicing:
- Add Invoice and VendorInvoiceSettings database models
- Full EU VAT support (27 countries, OSS, B2B reverse charge)
- Invoice PDF generation with WeasyPrint + Jinja2 templates
- Vendor invoice API endpoints for settings, creation, PDF download

Subscription Tiers:
- Add VendorSubscription model with 4 tiers (Essential/Professional/Business/Enterprise)
- Tier limit enforcement for orders, products, team members
- Feature gating based on subscription tier
- Automatic trial subscription creation for new vendors
- Integrate limit checks into order creation (direct and Letzshop sync)

Marketing:
- Update pricing documentation with 4-tier structure
- Revise back-office positioning strategy
- Update homepage with Veeqo-inspired Letzshop-focused messaging

🤖 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

@@ -21,6 +21,7 @@ from . import (
     dashboard,
     info,
     inventory,
+    invoices,
     letzshop,
     marketplace,
     media,
@@ -59,6 +60,7 @@ router.include_router(settings.router, tags=["vendor-settings"])
 router.include_router(products.router, tags=["vendor-products"])
 router.include_router(orders.router, tags=["vendor-orders"])
 router.include_router(order_item_exceptions.router, tags=["vendor-order-exceptions"])
+router.include_router(invoices.router, tags=["vendor-invoices"])
 router.include_router(customers.router, tags=["vendor-customers"])
 router.include_router(team.router, tags=["vendor-team"])
 router.include_router(inventory.router, tags=["vendor-inventory"])