S13: Remove a berth tenant

[xishell]

Persona

Harbor master + departing or removed berth tenant

User outcome

As a harbor master, I remove a tenant from a berth so the berth becomes available again — for example when a tenant leaves the harbor, is suspended, or is reassigned after a dispute. The removed tenant gets a notification so they know the change happened.

The backend already supports the removal (DELETE /api/berths/{berth_id}/assignment), but no UI exposes it, the removed user isn't notified, and role state isn't reconciled. This story closes those gaps.

Out of scope: tenant self-departure (S10 covers booking + self-service availability windows; tenant-initiated departure could be a future story).

Acceptance Criteria

• Harbor master sees a "Remove tenant" action on assigned berths in the berth detail panel
• Confirm gate on the action ("Remove $email from berth X?") prevents accidental removal
• On confirm, the assignment row is deleted and the berth status returns to free
• Removed tenant receives a transactional email ("Your berth assignment at $harbor has ended")
• If the removed user has no remaining berth assignments, their role downgrades from spot-owner to visitor
• Removal emits an audit event (assignment_removed) visible in the activity log
• Tests cover: happy path, role downgrade, role retention when user has another berth, audit event written