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/__init__.py

@@ -20,6 +20,7 @@ from . import (
     content_pages,
     customers,
     dashboard,
+    features,
     info,
     inventory,
     invoices,
@@ -36,6 +37,7 @@ from . import (
     profile,
     settings,
     team,
+    usage,
 )
 
 # Create vendor router
@@ -77,6 +79,8 @@ router.include_router(notifications.router, tags=["vendor-notifications"])
 router.include_router(messages.router, tags=["vendor-messages"])
 router.include_router(analytics.router, tags=["vendor-analytics"])
 router.include_router(billing.router, tags=["vendor-billing"])
+router.include_router(features.router, tags=["vendor-features"])
+router.include_router(usage.router, tags=["vendor-usage"])
 
 # Content pages management
 router.include_router(