fix(relay): emit participant-addressed DM discovery

[ScaleLeanChris]

Summary

• emit the relay-confirmed kind:41001 event that buzz dms list already queries
• add d and h channel tags plus a p tag for every DM participant
• keep the event channel-scoped so the existing private-channel read gate applies
• emit on both create and re-open so older DMs self-heal
• emit from the add-member path that creates immutable group DMs
• use the channel creation timestamp so repeated opens produce one stable event id

Root cause

The CLI discovery contract existed, but the relay had no kind:41001 producer. The relay emitted kind:39000 channel metadata and membership notifications, so the conversation appeared in buzz channels list while buzz dms list always returned an empty result.

Verification

• extended the existing real-relay DM discovery test to query kind:41001 as both the creator and recipient
• adds a third identity through kind:41011 and proves the resulting group DM is discoverable by its new participant
• asserts all participant p tags, the d and h channel tags, and one event after a recipient re-open
• cargo fmt --all
• git diff --check
• fork CI: Rust lint, unit tests, security, Windows, both Linux cross-compiles, Backend Integration, and the full Relay E2E suite pass

The fork-only macOS desktop build fails before compiling Buzz because its mesh-llm checkout is unavailable. The same environment failure occurs on the unchanged mobile test branch and is unrelated to this relay diff.

Fixes #2897

The relay already knows every participant, so DM discovery should not end as “Return to Sender.”