Add stackable add-ons with compounding integer features

- Add is_stackable field to AddOnProduct model for add-ons that can be
  purchased multiple times
- Add quantity field to SubscriptionAddOn for tracking purchase count
- Update EntitlementService to ADD integer add-on values to base plan
  (instead of max) and multiply by quantity for stackable add-ons
- Add feature selection to AddOnEditorModal using FeaturePicker component
- Add AddOnFeatureSerializer for nested feature CRUD on add-ons
- Fix Create Add-on button styling to use solid blue (was muted outline)
- Widen billing sidebar from 320px to 384px to prevent text wrapping

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/hooks/useBillingAdmin.ts

@@ -175,6 +175,20 @@ export interface PlanCreate {
   max_custom_domains?: number;
 }
 
+export interface AddOnFeature {
+  id: number;
+  feature: Feature;
+  bool_value: boolean | null;
+  int_value: number | null;
+  value: boolean | number | null;
+}
+
+export interface AddOnFeatureWrite {
+  feature_code: string;
+  bool_value?: boolean | null;
+  int_value?: number | null;
+}
+
 export interface AddOnProduct {
   id: number;
   code: string;
@@ -184,7 +198,9 @@ export interface AddOnProduct {
   price_one_time_cents: number;
   stripe_product_id: string;
   stripe_price_id: string;
+  is_stackable: boolean;
   is_active: boolean;
+  features: AddOnFeature[];
 }
 
 export interface AddOnProductCreate {
@@ -195,7 +211,9 @@ export interface AddOnProductCreate {
   price_one_time_cents?: number;
   stripe_product_id?: string;
   stripe_price_id?: string;
+  is_stackable?: boolean;
   is_active?: boolean;
+  features?: AddOnFeatureWrite[];
 }
 
 // Grandfathering response when updating a version with subscribers