feat(messaging): Add broadcast messaging system for owners and managers

- Add BroadcastMessage and MessageRecipient models for sending messages to groups or individuals
- Add Messages page with compose form and sent messages list
- Support targeting by role (owners, managers, staff, customers) or individual users
- Add can_send_messages permission (owners always, managers by default with revocable permission)
- Add autofill search dropdown with infinite scroll for selecting individual recipients
- Add staff permission toggle for managers' messaging access
- Integrate Messages link in sidebar for users with permission

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

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

smoothschedule/config/settings/multitenancy.py

@@ -66,6 +66,9 @@ TENANT_APPS = [
     'smoothschedule.scheduling.schedule',  # Resource scheduling with configurable concurrency
     'smoothschedule.scheduling.contracts',  # Contract/e-signature system
 
+    # Communication Domain (tenant-isolated)
+    'smoothschedule.communication.messaging',  # Broadcast messaging system
+
     # Commerce Domain (tenant-isolated)
     'smoothschedule.commerce.payments',  # Stripe Connect payments bridge
 ]