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>

models/database/vendor.py

@@ -143,6 +143,29 @@ class Vendor(Base, TimestampMixin):
         cascade="all, delete-orphan",
     )
 
+    # Invoice settings (one-to-one)
+    invoice_settings = relationship(
+        "VendorInvoiceSettings",
+        back_populates="vendor",
+        uselist=False,
+        cascade="all, delete-orphan",
+    )
+
+    # Invoices (one-to-many)
+    invoices = relationship(
+        "Invoice",
+        back_populates="vendor",
+        cascade="all, delete-orphan",
+    )
+
+    # Subscription (one-to-one)
+    subscription = relationship(
+        "VendorSubscription",
+        back_populates="vendor",
+        uselist=False,
+        cascade="all, delete-orphan",
+    )
+
     domains = relationship(
         "VendorDomain",
         back_populates="vendor",