fix: use COALESCE in audit JPQL to prevent lower(bytea) error

Hibernate passes null search param as bytea type, causing PostgreSQL
to fail on LOWER(bytea). COALESCE converts null to empty string.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/java/net/siegeln/cameleer/saas/audit/AuditRepository.java

@@ -26,8 +26,8 @@ public interface AuditRepository extends JpaRepository<AuditEntity, UUID> {
           AND (:from IS NULL OR a.createdAt >= :from)
           AND (:to IS NULL OR a.createdAt <= :to)
           AND (:search IS NULL
-               OR LOWER(a.actorEmail) LIKE LOWER(CONCAT('%', :search, '%'))
-               OR LOWER(a.resource) LIKE LOWER(CONCAT('%', :search, '%')))
+               OR LOWER(a.actorEmail) LIKE LOWER(CONCAT('%', COALESCE(:search, ''), '%'))
+               OR LOWER(a.resource) LIKE LOWER(CONCAT('%', COALESCE(:search, ''), '%')))
         ORDER BY a.createdAt DESC
         """)
     Page<AuditEntity> findFiltered(