feat(relay): deliver pubkey mentions to relay companions

[jsibbison-square]

Summary

Deliver mentions of registered pubkeys to deployment-global operator listeners.

• Add NIP-98 APIs to register and remove target pubkeys.
• Enqueue matching notifications transactionally during event ingest.
• Deliver through a bounded retrying outbox worker and clean registrations daily.
• Keep this path app-managed without database triggers, functions, foreign keys, or a match queue.

Changed behavior

Event inserts, threads, reactions, and guarded writes now commit event_mentions and listener outbox rows inside the event transaction. Previously, mention indexing ran after the event/reaction commit and failures were warn-only; those failures now reject and roll back the event, including ordinary p-tagged events and reactions.

Testing

• cargo fmt --all
• cargo test -p buzz-db --lib
• cargo test -p buzz-relay --lib config::tests::operator_listener_routes
• cargo check -p buzz-db -p buzz-relay

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 77729abfb692b25a0f4ec4a69add86af2e32c0dd...723aed3b1c6ea1dd0a1a24ca54a031481ed599d0.
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