feat: schedule due processing from next dueAt instead of polling

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

ordo-core/src/main/java/com/jetlumen/ordo/core/DefaultOrdoEngine.java

@@ -345,6 +345,11 @@ public final class DefaultOrdoEngine implements OrdoEngine {
         return processed;
     }
 
+    @Override
+    public synchronized Optional<Instant> nextDueAt() {
+        return taskRepository.findNextDueAt();
+    }
+
     private boolean escalateDueTask(ApprovalTask overdue, Instant now, List<PendingAction> queued,
                                     List<ProcessEvent> events) {
         if (!taskRepository.claimIfDue(overdue.id(), overdue.assignee(), now)) {

ordo-core/src/main/java/com/jetlumen/ordo/core/InMemoryOrdoEngine.java

@@ -23,6 +23,7 @@ import com.jetlumen.ordo.core.repository.InMemoryProcessHistoryRepository;
 import com.jetlumen.ordo.core.repository.InMemoryProcessInstanceRepository;
 
 import java.time.Clock;
+import java.time.Instant;
 import java.util.List;
 import java.util.Optional;
 
@@ -181,4 +182,9 @@ public final class InMemoryOrdoEngine implements OrdoEngine {
     public int processDue(int limit) {
         return delegate.processDue(limit);
     }
+
+    @Override
+    public Optional<Instant> nextDueAt() {
+        return delegate.nextDueAt();
+    }
 }

ordo-core/src/main/java/com/jetlumen/ordo/core/repository/InMemoryApprovalTaskRepository.java

@@ -113,6 +113,15 @@ public final class InMemoryApprovalTaskRepository implements ApprovalTaskReposit
                 .toList();
     }
 
+    @Override
+    public synchronized Optional<Instant> findNextDueAt() {
+        return tasks.values().stream()
+                .filter(task -> task.status() == TaskStatus.PENDING)
+                .map(ApprovalTask::dueAt)
+                .filter(Objects::nonNull)
+                .min(Comparator.naturalOrder());
+    }
+
     @Override
     public synchronized boolean claimIfDue(String taskId, String expectedAssignee, Instant now) {
         ApprovalTask current = tasks.get(taskId);