feat: Quota overage system, updated tier pricing, and communication addons

Quota Overage System:
- Add QuotaOverage model for tracking resource/user quota overages
- Implement 30-day grace period with email notifications (immediate, 7-day, 1-day)
- Add QuotaWarningBanner component in BusinessLayout
- Add QuotaSettings page for managing overages and archiving resources
- Add Celery tasks for automated quota checks and expiration handling
- Add quota management API endpoints

Updated Tier Pricing (Stripe: 2.9% + $0.30):
- Free: No payments (requires addon)
- Starter: 4% + $0.40
- Professional: 3.5% + $0.35
- Business: 3.25% + $0.32
- Enterprise: 3% + $0.30

New Subscription Addons:
- Online Payments ($5/mo + 5% + $0.50) - for Free tier
- SMS Notifications ($10/mo) - enables SMS reminders
- Masked Calling ($15/mo) - enables anonymous calling

BusinessEditModal Improvements:
- Increased width to match PlanModal (max-w-3xl)
- Added all tier options with auto-update on tier change
- Added limits configuration and permissions sections

Backend Fixes:
- Fixed SubscriptionPlan serializer to include all communication fields
- Allow blank business_tier for addon plans
- Added migration for business_tier field changes

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

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

frontend/src/pages/platform/PlatformSettings.tsx

@@ -785,12 +785,14 @@ const PlanModal: React.FC<PlanModalProps> = ({ plan, onSave, onClose, isLoading
       can_api_access: false,
       can_use_masked_phone_numbers: false,
     },
+    // Default transaction fees: Stripe charges 2.9% + $0.30, so we need to charge more
+    // Recommended: FREE 5%+50¢, STARTER 4%+40¢, PROFESSIONAL 3.5%+35¢, ENTERPRISE 3%+30¢
     transaction_fee_percent: plan?.transaction_fee_percent
       ? parseFloat(plan.transaction_fee_percent)
-      : 0,
+      : 4.0, // Default 4% for new plans
     transaction_fee_fixed: plan?.transaction_fee_fixed
       ? parseFloat(plan.transaction_fee_fixed)
-      : 0,
+      : 40, // Default 40 cents for new plans
     // Communication pricing
     sms_enabled: plan?.sms_enabled ?? false,
     sms_price_per_message_cents: plan?.sms_price_per_message_cents ?? 3,
@@ -1048,6 +1050,9 @@ const PlanModal: React.FC<PlanModalProps> = ({ plan, onSave, onClose, isLoading
                 />
               </div>
             </div>
+            <p className="text-xs text-amber-600 dark:text-amber-400 bg-amber-50 dark:bg-amber-900/20 p-2 rounded">
+              Stripe charges 2.9% + $0.30 per transaction + $2/mo per connected account. Recommended: STARTER 4%+40¢, PROFESSIONAL 3.5%+35¢, ENTERPRISE 3%+30¢. Consider a payments add-on ($5/mo) for FREE tier users.
+            </p>
           </div>
 
           {/* Communication Pricing */}
@@ -1258,7 +1263,7 @@ const PlanModal: React.FC<PlanModalProps> = ({ plan, onSave, onClose, isLoading
             <div className="grid grid-cols-3 gap-4">
               <div>
                 <label className="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-1">
-                  Max Users
+                  Max Additional Users
                 </label>
                 <input
                   type="number"