feat: schedule due processing from next dueAt instead of polling

Co-authored-by: Cursor <cursoragent@cursor.com>

ordo-api/src/main/java/com/jetlumen/ordo/api/OrdoEngine.java

@@ -11,6 +11,7 @@ import com.jetlumen.ordo.api.runtime.ProcessContext;
 import com.jetlumen.ordo.api.runtime.ProcessEvent;
 import com.jetlumen.ordo.api.runtime.ProcessInstance;
 
+import java.time.Instant;
 import java.util.List;
 import java.util.Optional;
 
@@ -40,6 +41,9 @@ public interface OrdoEngine {
     /** Claims and processes up to {@code limit} overdue pending tasks. {@code limit} must be positive. */
     int processDue(int limit);
 
+    /** Earliest {@code dueAt} among pending tasks, or empty if none are scheduled. */
+    Optional<Instant> nextDueAt();
+
     default ProcessInstance withdraw(String instanceId, String actor) {
         return withdraw(instanceId, actor, null);
     }

ordo-api/src/main/java/com/jetlumen/ordo/api/repository/ApprovalTaskRepository.java

@@ -43,6 +43,9 @@ public interface ApprovalTaskRepository {
 
     List<ApprovalTask> findDuePending(Instant now, int limit);
 
+    /** Earliest {@code dueAt} among pending tasks, or empty if none are scheduled. */
+    Optional<Instant> findNextDueAt();
+
     boolean claimIfDue(String taskId, String expectedAssignee, Instant now);
 
     /** Paginated, filterable query; results are ordered newest-first (created_at desc). */