feat(audit): add audit_log infrastructure and instrument AnnotationService

- V46 migration: audit_log table with indexes and append-only REVOKE
- audit/ package: AuditKind enum (with Javadoc payloads), AuditLog entity,
  AuditLogRepository, AuditService (@Async on dedicated auditExecutor)
- AsyncConfig: auditExecutor with CallerRunsPolicy and queueCapacity 50
- AnnotationService: ANNOTATION_CREATED on createAnnotation() only,
  deferred via afterCommit() when inside a transaction

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

backend/src/main/java/org/raddatz/familienarchiv/config/AsyncConfig.java

@@ -23,4 +23,15 @@ public class AsyncConfig {
         executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
         return executor;
     }
+
+    @Bean("auditExecutor")
+    public Executor auditExecutor() {
+        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+        executor.setCorePoolSize(1);
+        executor.setMaxPoolSize(2);
+        executor.setQueueCapacity(50);
+        executor.setThreadNamePrefix("Audit-");
+        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
+        return executor;
+    }
 }