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>

Makefile

@@ -44,7 +44,7 @@ setup: install-all migrate-up init-prod
 # =============================================================================
 
 dev:
-	$(PYTHON) -m uvicorn main:app --reload --host 0.0.0.0 --port 8000
+	$(PYTHON) -m uvicorn main:app --reload --host 0.0.0.0 --port 9999
 
 # =============================================================================
 # DATABASE MIGRATIONS
@@ -274,7 +274,7 @@ qa: format lint arch-check test-coverage docs-check
 
 docs-serve:
 	@echo "Starting documentation server..."
-	$(PYTHON) -m mkdocs serve --dev-addr=0.0.0.0:8001
+	$(PYTHON) -m mkdocs serve --dev-addr=0.0.0.0:9991
 
 docs-build:
 	@echo "Building documentation..."