Say Hi To User

Silent ice-breaker DM.

Behaviour:

1. Self-hi is rejected.
2. Block check (via _can_dm) fails closed with generic copy.
3. Reuse or create a direct thread between sender and target.
4. Insert one message: "{sender_name} said hi." with is_icebreaker=true.
5. Mark the sender's participant row hidden_until_reply=true so the thread stays out of their inbox list until the target replies.
6. Write a row to user_say_hi_dismissals so the Active People rail doesn't pitch the same target again to this sender.
7. Notify recipient through the existing _notify_new_message path (their badge ticks; sound is gated by their availability prefs).
8. Return a warm, generic confirmation. NEVER expose mute/block.

Response shape mirrors the spec: { ok, thread_id, message_id, delivery, recipient: {...}, feedback }