feat(relay): allow channel admins to edit posts in their channel with edited_by provenance

[pete-lucern]

Problem

Channel admins could delete and edit channel metadata, but kind 40003 message edits were limited to the original author (or the existing agent-owner exception). Projected tile/status-board posts therefore could not be maintained by a designated channel administrator.

Change

• Allow the target message author, or an owner/admin member of the target event's channel, to edit a message. The existing same-channel target check remains enforced; admins from another channel are rejected.
• Require delegated edits to carry a signed edited_by tag naming the editor, leaving the original event author unchanged. The SDK, CLI, and desktop builders now add this provenance.
• Emit the existing relay audit path plus a durable kind-40099 system message with type message_edited_by_admin, actor, target event ID, and channel. Desktop renders it as “ edited a message”.
• No feature flag or schema change was added: this uses the existing channel-role lookup, audit queue, and system-message path, with admin edit enabled by default as requested.

Tests

• Focused relay edit tests: 6 passed, 0 failed.
• cargo clippy -p buzz-relay --lib -- -D warnings: passed.
• cargo check -p buzz-cli: passed.
• cargo test -p buzz-relay --lib: 1063 passed, 7 failed, 122 ignored. The seven failures are unrelated environment-sensitive tests: six require a local Postgres buzz role, and one telemetry test depends on process-global tracing state.

Closest existing PR:

None found for channel-admin message editing. gh pr list -R block/buzz --search "edit admin" returned no directly relevant match; the closest-looking result was #5243, which concerns DM participant renaming rather than channel message authorization.

[github-actions]

🔐 Codex Security Review

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