feat: 提交流程中心后端代码

Spring Boot 流程引擎服务：流程定义/实例/任务、审批人解析、
回调分发、oapi 接口及单元测试。
保留远程 .gitea/workflows/build.yml 与 docker/Dockerfile，
新增 .gitignore 排除 target/ 构建产物。

src/main/java/com/yunhai/flowcenter/engine/FlowAdvanceService.java

@@ -0,0 +1,62 @@
+package com.yunhai.flowcenter.engine;
+
+import com.yunhai.flowcenter.entity.FlwDef;
+import com.yunhai.flowcenter.entity.FlwInstance;
+import com.yunhai.flowcenter.enums.FlwEnums;
+import com.yunhai.flowcenter.repository.FlwDefRepository;
+import com.yunhai.flowcenter.repository.FlwInstanceRepository;
+import com.yunhai.flowcenter.repository.FlwTaskRepository;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Propagation;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * 节点放行推进（AFTER_COMMIT 监听器经代理调用本 bean，REQUIRES_NEW 生效——
+ * 事务已提交后的同步监听阶段没有活动事务，@Modifying 批量语句必须有新事务承载）：
+ * 该节点待办清零且办件在途才推进；并发冲突由 flw_instance.version 乐观锁暴露，监听器层负责重试。
+ */
+@Slf4j
+@Service
+@RequiredArgsConstructor
+public class FlowAdvanceService {
+
+    private final FlwInstanceRepository instanceRepository;
+    private final FlwDefRepository defRepository;
+    private final FlwTaskRepository taskRepository;
+    private final FlowEngine flowEngine;
+
+    /** @return 是否实际推进了 */
+    @Transactional(propagation = Propagation.REQUIRES_NEW)
+    public boolean tryAdvance(Long instanceId, String nodeId) {
+        FlwInstance instance = instanceRepository.findById(instanceId).orElse(null);
+        if (instance == null
+                || instance.getStatus() != FlwEnums.InstanceStatus.APPROVING.getCode()) {
+            return false;
+        }
+        // 已推进过的节点不再放行（currentNodes 由引擎在放行时移除，乐观锁保证互斥）
+        String current = instance.getCurrentNodes();
+        boolean active = current != null && java.util.Arrays.stream(current.split(","))
+                .anyMatch(nodeId::equals);
+        if (!active) {
+            return false;
+        }
+        long pending = taskRepository.findByInstanceIdAndStatus(instanceId,
+                FlwEnums.TaskStatus.PENDING.getCode()).stream()
+                .filter(t -> nodeId.equals(t.getNodeId()))
+                .count();
+        if (pending > 0) {
+            return false;
+        }
+        FlwDef def = defRepository.findById(instance.getDefId()).orElse(null);
+        if (def == null) {
+            log.error("[flow-center] 办件定义缺失，推进中止：instance={} defId={}",
+                    instanceId, instance.getDefId());
+            return false;
+        }
+        flowEngine.continueAdvance(instance, def, nodeId);
+        instanceRepository.save(instance);
+        return true;
+    }
+}