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/layouts/SettingsLayout.tsx

@@ -20,6 +20,7 @@ import {
   Phone,
   CreditCard,
   Webhook,
+  AlertTriangle,
 } from 'lucide-react';
 import {
   SettingsSidebarSection,
@@ -137,6 +138,12 @@ const SettingsLayout: React.FC = () => {
               label={t('settings.billing.title', 'Plan & Billing')}
               description={t('settings.billing.description', 'Subscription, invoices')}
             />
+            <SettingsSidebarItem
+              to="/settings/quota"
+              icon={AlertTriangle}
+              label={t('settings.quota.title', 'Quota Management')}
+              description={t('settings.quota.description', 'Usage limits, archiving')}
+            />
           </SettingsSidebarSection>
         </nav>
       </aside>