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/test/java/com/yunhai/flowcenter/stub/StubUserClient.java

@@ -0,0 +1,140 @@
+package com.yunhai.flowcenter.stub;
+
+import com.yunhai.platform.openapi.client.OpenApiUserClient;
+import com.yunhai.platform.openapi.model.SysUserGroupDeleteItem;
+import com.yunhai.platform.openapi.model.SysUserGroupStateRequest;
+import com.yunhai.platform.openapi.model.SysUserMoveRequest;
+import com.yunhai.platform.openapi.model.SysUserRequest;
+import com.yunhai.platform.openapi.model.SysUserTransferSchoolRequest;
+import com.yunhai.platform.utils.model.BoaGroup;
+import com.yunhai.platform.utils.model.BoaModule;
+import com.yunhai.platform.utils.model.BoaRoleInfo;
+import com.yunhai.platform.utils.model.BoaUser;
+
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * UserBase 客户端测试 stub：getUserById/getUserInfo 返回 school=1（测试学校）的用户，
+ * 供 OapiFlowService submitterId 本校校验（测试统一用 schId=1）；其余端点不参与 M1 测试。
+ */
+public class StubUserClient implements OpenApiUserClient {
+
+    public static final long TEST_SCH_ID = 1L;
+
+    @Override
+    public BoaUser getUserById(String userId) {
+        return buildUser(Integer.valueOf(userId));
+    }
+
+    @Override
+    public BoaUser getUserInfo(int userId) {
+        return buildUser(userId);
+    }
+
+    private static BoaUser buildUser(int userId) {
+        BoaUser user = new BoaUser(userId, "user" + userId, "用户" + userId);
+        user.setSchool(new BoaGroup((int) TEST_SCH_ID, "测试学校"));
+        return user;
+    }
+
+    @Override
+    public BoaUser getUserByLoginName(String loginName) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public BoaUser getUserByPhone(String phone) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public BoaUser getUserInfo(String loginName) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public List<BoaModule> getUserModule(int userId, Integer schoolId) {
+        return Collections.emptyList();
+    }
+
+    @Override
+    public BoaUser addUser(SysUserRequest request) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public BoaUser editUser(int id, SysUserRequest request) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void deleteUser(int id, int groupId, int schoolId) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void deleteUsers(List<SysUserGroupDeleteItem> items) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public List<BoaUser> batchAddUsers(List<SysUserRequest> requests) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public BoaUser moveUserToGroup(SysUserMoveRequest request) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public List<BoaRoleInfo> getUserRoleList(int userId) {
+        return Collections.emptyList();
+    }
+
+    @Override
+    public BoaRoleInfo getCurrentIdentity(int userId) {
+        return null;
+    }
+
+    @Override
+    public BoaRoleInfo switchIdentity(int userId, int roleId, int schoolId) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void updateUserPassword(int userId, String oldPassword, String newPassword) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public String updateUserProfilePicture(int userId, byte[] fileBytes, String filename) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void setUserGroupState(SysUserGroupStateRequest request) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public Integer getUserGroupState(int userId, int groupId, int schoolId) {
+        return 0;
+    }
+
+    @Override
+    public BoaUser transferUserToSchool(SysUserTransferSchoolRequest request) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void changeUserStatus(int userId, boolean disabled) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void batchChangeUserStatus(List<Integer> userIds, boolean disabled) {
+        throw new UnsupportedOperationException();
+    }
+}