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>

frontend/src/components/StaffPermissions.tsx

@@ -68,6 +68,15 @@ export const PERMISSION_CONFIGS: PermissionConfig[] = [
     defaultValue: false,
     roles: ['manager'],
   },
+  {
+    key: 'can_send_messages',
+    labelKey: 'staff.canSendMessages',
+    labelDefault: 'Can send broadcast messages',
+    hintKey: 'staff.canSendMessagesHint',
+    hintDefault: 'Send messages to groups of staff and customers',
+    defaultValue: true,
+    roles: ['manager'],
+  },
   // Staff-only permissions
   {
     key: 'can_view_all_schedules',