Slack: deprovision user to release paid seat

[LuD1161]

Goal: Ensure the user no longer consumes a paid seat.
Tasks:

• Research which Slack action actually frees a seat (deactivate vs remove; SCIM vs admin API).

• Identify API endpoint(s) to deprovision user.

• Input: email or slack_user_id

• Steps: resolve user → deprovision → transfer private channel ownership (optional).

• Verify billing seat count drops.

• Emit audit JSON (userId, action, result).
  Acceptance:

• Dry-run supported.

• Idempotent re-runs.

• Unit tests for deactivate + transfer path.