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/__init__.py

@@ -19,6 +19,12 @@ from .company import Company
 from .content_page import ContentPage
 from .customer import Customer, CustomerAddress
 from .inventory import Inventory
+from .invoice import (
+    Invoice,
+    InvoiceStatus,
+    VATRegime,
+    VendorInvoiceSettings,
+)
 from .letzshop import (
     LetzshopFulfillmentQueue,
     LetzshopHistoricalImportJob,
@@ -44,6 +50,12 @@ from .order import Order, OrderItem
 from .order_item_exception import OrderItemException
 from .product import Product
 from .product_translation import ProductTranslation
+from .subscription import (
+    SubscriptionStatus,
+    TierCode,
+    TIER_LIMITS,
+    VendorSubscription,
+)
 from .test_run import TestCollection, TestResult, TestRun
 from .user import User
 from .vendor import Role, Vendor, VendorUser
@@ -95,6 +107,11 @@ __all__ = [
     "MarketplaceImportError",
     # Inventory
     "Inventory",
+    # Invoicing
+    "Invoice",
+    "InvoiceStatus",
+    "VATRegime",
+    "VendorInvoiceSettings",
     # Orders
     "Order",
     "OrderItem",
@@ -104,6 +121,11 @@ __all__ = [
     "LetzshopFulfillmentQueue",
     "LetzshopSyncLog",
     "LetzshopHistoricalImportJob",
+    # Subscription
+    "VendorSubscription",
+    "SubscriptionStatus",
+    "TierCode",
+    "TIER_LIMITS",
     # Messaging
     "Conversation",
     "ConversationParticipant",