Admin Add Thread Participant

Add a handle as a participant to an existing thread.

Use case: surface a thread in someone's inbox without rewriting the message history. The original messages stay immutable; the new participant gains read/reply visibility going forward. Brian m_… 2026-04-30 — adding himself to threads Leslie originated to Joe so he can engage without tampering with the audit trail (Concept 1 integrity preserved).

Body: { "handle": "brian@tandem-cc.com", # required "role": "participant", # optional, default "participant" "added_by": "brian@tandem-cc.com" # optional, defaults to handle (self-add) }

Idempotent — ON CONFLICT DO NOTHING.