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/services/letzshop/order_service.py

@@ -15,6 +15,7 @@ from sqlalchemy import String, and_, func, or_
 from sqlalchemy.orm import Session
 
 from app.services.order_service import order_service as unified_order_service
+from app.services.subscription_service import subscription_service
 from models.database.letzshop import (
     LetzshopFulfillmentQueue,
     LetzshopHistoricalImportJob,
@@ -792,6 +793,7 @@ class LetzshopOrderService:
             "updated": 0,
             "skipped": 0,
             "errors": 0,
+            "limit_exceeded": 0,
             "products_matched": 0,
             "products_not_found": 0,
             "eans_processed": set(),
@@ -800,6 +802,10 @@ class LetzshopOrderService:
             "error_messages": [],
         }
 
+        # Get subscription usage upfront for batch efficiency
+        usage = subscription_service.get_usage(self.db, vendor_id)
+        orders_remaining = usage.orders_remaining  # None = unlimited
+
         for i, shipment in enumerate(shipments):
             shipment_id = shipment.get("id")
             if not shipment_id:
@@ -844,11 +850,24 @@ class LetzshopOrderService:
                 else:
                     stats["skipped"] += 1
             else:
+                # Check tier limit before creating order
+                if orders_remaining is not None and orders_remaining <= 0:
+                    stats["limit_exceeded"] += 1
+                    stats["error_messages"].append(
+                        f"Shipment {shipment_id}: Order limit reached"
+                    )
+                    continue
+
                 # Create new order using unified service
                 try:
                     self.create_order(vendor_id, shipment)
                     self.db.commit()  # noqa: SVC-006 - background task needs incremental commits
                     stats["imported"] += 1
+
+                    # Decrement remaining count for batch efficiency
+                    if orders_remaining is not None:
+                        orders_remaining -= 1
+
                 except Exception as e:
                     self.db.rollback()  # Rollback failed order
                     stats["errors"] += 1