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/routes/admin_pages.py

@@ -1286,3 +1286,27 @@ async def admin_platform_health(
             "user": current_user,
         },
     )
+
+
+# ============================================================================
+# FEATURE MANAGEMENT ROUTES
+# ============================================================================
+
+
+@router.get("/features", response_class=HTMLResponse, include_in_schema=False)
+async def admin_features_page(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render feature management page.
+    Shows all features with tier assignments and allows editing.
+    """
+    return templates.TemplateResponse(
+        "admin/features.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )