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/pom.xml

@@ -103,6 +103,11 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-webmvc-test</artifactId>
 			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>org.awaitility</groupId>
+			<artifactId>awaitility</artifactId>
+			<scope>test</scope>
 		</dependency>
 				<!-- Excel Bearbeitung (Apache POI) -->
 		<dependency>