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>

models/database/__init__.py

@@ -19,6 +19,7 @@ from .company import Company
 from .content_page import ContentPage
 from .customer import Customer, CustomerAddress
 from .email import EmailCategory, EmailLog, EmailStatus, EmailTemplate
+from .feature import Feature, FeatureCategory, FeatureCode, FeatureUILocation
 from .inventory import Inventory
 from .invoice import (
     Invoice,
@@ -108,6 +109,11 @@ __all__ = [
     "EmailLog",
     "EmailStatus",
     "EmailTemplate",
+    # Features
+    "Feature",
+    "FeatureCategory",
+    "FeatureCode",
+    "FeatureUILocation",
     # Product - Enums
     "ProductType",
     "DigitalDeliveryMethod",