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/templates/vendor/base.html

@@ -62,7 +62,13 @@
     <!-- 5. FIFTH: API Client (depends on Utils) -->
     <script src="{{ url_for('static', path='shared/js/api-client.js') }}"></script>
 
-    <!-- 6. SIXTH: Alpine.js v3 with CDN fallback (with defer) -->
+    <!-- 6. SIXTH: Feature Store (depends on API Client, registers with Alpine) -->
+    <script src="{{ url_for('static', path='shared/js/feature-store.js') }}"></script>
+
+    <!-- 7. SEVENTH: Upgrade Prompts (depends on API Client, registers with Alpine) -->
+    <script src="{{ url_for('static', path='shared/js/upgrade-prompts.js') }}"></script>
+
+    <!-- 8. EIGHTH: Alpine.js v3 with CDN fallback (with defer) -->
     <script>
     (function() {
         var script = document.createElement('script');
@@ -81,7 +87,7 @@
     })();
     </script>
 
-    <!-- 7. LAST: Page-specific scripts -->
+    <!-- 9. LAST: Page-specific scripts -->
     {% block extra_scripts %}{% endblock %}
 </body>
 </html>