Support multiple assignees and CC recipients on cleaning tasks

Hosts should be able to include multiple teammates on a single cleaning task.

This would support:
• assigning more than one cleaner when a large home requires a crew;
• adding a secondary assignee or helper without replacing the primary cleaner; and
• CCing managers or other members of the same team on task-created, updated, and cancelled notifications.

Today, task notifications go only to the primary assignee, which makes it difficult to coordinate larger turnovers and keep an operations manager informed. Ideally, one teammate would remain the primary owner while additional teammates could be added as assignees, helpers, or notification-only recipients, with clear permissions for accepting or declining the task.

This would improve coverage, accountability, and communication without requiring external forwarding or duplicate manual messages.