feat: add comprehensive tier-based feature management system

Implement database-driven feature gating with contextual upgrade prompts:

- Add Feature model with 30 features across 8 categories
- Create FeatureService with caching for tier-based feature checking
- Add @require_feature decorator and RequireFeature dependency for backend enforcement
- Create vendor features API (6 endpoints) and admin features API
- Add Alpine.js feature store and upgrade prompts store for frontend
- Create Jinja macros: feature_gate, feature_locked, limit_warning, usage_bar
- Add usage API for tracking orders/products/team limits with upgrade info
- Fix Stripe webhook to create VendorAddOn records on addon purchase
- Integrate upgrade prompts into vendor dashboard with tier badge and usage bars

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/vendor/invoices.py

@@ -16,6 +16,12 @@ Endpoints:
 - POST /invoices/settings - Create invoice settings
 - PUT /invoices/settings - Update invoice settings
 - GET /invoices/stats - Get invoice statistics
+
+Feature Requirements:
+- invoice_lu: Basic Luxembourg invoicing (Essential tier)
+- invoice_eu_vat: EU VAT support (Professional tier)
+- invoice_bulk: Bulk invoicing (Business tier)
+- accounting_export: Export to accounting software (Business tier)
 """
 
 import logging
@@ -27,6 +33,7 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
+from app.core.feature_gate import RequireFeature
 from app.exceptions.invoice import (
     InvoiceNotFoundException,
     InvoicePDFGenerationException,
@@ -34,6 +41,7 @@ from app.exceptions.invoice import (
     InvoiceSettingsNotFoundException,
 )
 from app.services.invoice_service import invoice_service
+from models.database.feature import FeatureCode
 from models.database.user import User
 from models.schema.invoice import (
     InvoiceCreate,
@@ -61,11 +69,13 @@ logger = logging.getLogger(__name__)
 def get_invoice_settings(
     current_user: User = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
+    _: None = Depends(RequireFeature(FeatureCode.INVOICE_LU)),
 ):
     """
     Get vendor invoice settings.
 
     Returns null if settings not yet configured.
+    Requires: invoice_lu feature (Essential tier)
     """
     settings = invoice_service.get_settings(db, current_user.token_vendor_id)
     if settings: