fix(audit): submit afterCommit write to executor to avoid transaction sync conflict

AuditService.logAfterCommit() called writeLog() inline inside the afterCommit()
callback. At that point Spring's transaction synchronizations are still active on
the thread, so SimpleJpaRepository.save() throws IllegalStateException which the
catch block silently swallowed — leaving audit_log permanently empty.

Fix: submit writeLog() to auditExecutor so it runs on a fresh thread with no active
synchronization context. Also switch auditExecutor from CallerRunsPolicy to AbortPolicy
to prevent the bug from silently recurring when the queue fills under load.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V46__add_audit_log.sql

@@ -19,4 +19,7 @@ CREATE INDEX idx_audit_log_kind        ON audit_log (kind);
 
 -- Enforce append-only at the database layer: the application role may INSERT
 -- but must not UPDATE or DELETE audit rows.
+-- NOTE: This REVOKE is a no-op when the current user is the table owner.
+-- PostgreSQL owners retain all privileges regardless of REVOKE. The append-only
+-- guarantee is enforced at the application layer only.
 REVOKE UPDATE, DELETE ON audit_log FROM CURRENT_USER;