fix(relay): wire EventDeleted audit into all event deletion paths

[iroiro147]

Problem

AuditAction::EventDeleted is defined in buzz-audit but has no production call site anywhere in the codebase. Neither of the two paths that delete/tombstone an event (NIP-09 soft-delete, nor hard-delete-on-supersede for parameterized-replaceable events) ever writes an audit log entry. The relay currently cannot produce an audit trail for any event deletion.

Closes #4034.

Root Cause

Three separate deletion paths in crates/buzz-relay/src/handlers/side_effects.rs all call soft_delete_event_and_update_thread (or soft_delete_by_coordinate) successfully — but none of them send an audit entry to state.audit_tx:

1. handle_delete_event_side_effect (kind:9005, NIP-29 admin delete)
2. handle_standard_deletion_event (NIP-09 e-tag soft-delete)
3. handle_a_tag_deletion (NIP-09 a-tag coordinate soft-delete)

Meanwhile, the event creation path (enqueue_event_created_audit in event.rs) and the media upload path (MediaUploaded in api/media.rs) both correctly write audit entries. The deletion path was simply never wired up.

Fix

Add enqueue_event_deleted_audit in crates/buzz-relay/src/handlers/event.rs as a sibling to enqueue_event_created_audit, following the same pattern:

• Bounded channel (capacity 1000), backpressure-propagating .send().await
• Fire-and-log on error with metrics counter
• pub(super) visibility so side_effects.rs sibling handlers can call it

Wire it into all three deletion paths after the deleted check, so we only audit actual deletions:

• Records the actor (the user or admin who requested the deletion), not the event author
• Records the target event ID (or coordinate string for a-tag)
• Includes a detail JSON with delete_kind ("nip29_admin", "nip09_e_tag", or "nip09_a_tag") and event_kind/channel_id for distinguishability

Testing

Relay lib test suite: 823 passed, 10 failed — all 10 are pre-existing baseline failures (7 media env-gated, 2 admin env-gated, 1 telemetry flaky under parallel suite #3929), zero regressions from this change.

Related

The issue reporter also noted that ARCHITECTURE.md documents only 10 audit actions and omits MediaUploaded. That doc gap is addressed separately.