fix(audit): make delivery durable and tenant-fair

[TheSentinel454]

Why

The follow-up review on #6229 identified audit-delivery gaps under database timeouts, tenant contention, and worker teardown. In particular, a supported statement_timeout could lose an audit entry, one busy community could monopolize the bounded queue, and the idle-transaction test did not prove that PostgreSQL actually terminated and replaced the backend.

What

• replace the in-memory audit channel with a durable, per-community ordered outbox
• retain logical delivery keys so producer retries deduplicate both pending and delivered audit entries
• retry PostgreSQL lock and statement timeouts without blocking unrelated communities
• bound shutdown, abort timed-out workers, and resume durable work after restart
• fail closed when an event or media audit intent cannot be persisted
• add regression coverage for lock/statement timeouts, 1,001-item saturation, cross-community progress, cancellation, restart recovery, logical deduplication, and real idle-transaction backend replacement
• update the stale event-handler retry comment

Risk Assessment

Medium. This adds two community-scoped tables and changes audit delivery from process memory to PostgreSQL. The migration and deletion manifests are covered by schema tests, and the delivery path is protected by transactional enqueue/deduplication, per-community ordering, advisory locking, bounded retry backoff, and restart tests.

Verification

• cargo clippy -p buzz-db -p buzz-audit -p buzz-relay --all-targets -- -D warnings
• cargo test -p buzz-db --lib
• cargo test -p buzz-audit
• cargo test -p buzz-audit -- --ignored --nocapture --test-threads=1
• cargo test -p buzz-relay --lib
• ignored PostgreSQL idle-transaction replacement and fresh-migration tests
• repository pre-push hooks

References

• Follow-up to feat(db): configurable writer session timeouts (lock, idle-txn, statement) #6229
• Review feedback

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[github-actions]

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