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/admin/__init__.py

@@ -33,6 +33,7 @@ from . import (
     content_pages,
     customers,
     dashboard,
+    features,
     images,
     inventory,
     letzshop,
@@ -181,6 +182,9 @@ router.include_router(
 # Include subscription management endpoints
 router.include_router(subscriptions.router, tags=["admin-subscriptions"])
 
+# Include feature management endpoints
+router.include_router(features.router, tags=["admin-features"])
+
 
 # ============================================================================
 # Code Quality & Architecture