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/controller/oapi/OapiFlowController.java

@@ -0,0 +1,64 @@
+package com.yunhai.flowcenter.controller.oapi;
+
+import com.yunhai.flowcenter.model.OapiModels;
+import com.yunhai.flowcenter.service.OapiFlowService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestAttribute;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RestController;
+
+import java.util.List;
+
+/**
+ * 托管接入件（/oapi/flow/**，appKey 签名强制，V2.0 Part D #5 前置 M1）。
+ * 出入参逐字对齐 svc-hall FlowCenterClient（《流程中心M1开工说明.md》§5 契约锚点）：
+ * - GET  /oapi/flow/list?schId=                          → data=[{flowGuid,name,version,nodeSummary}]
+ * - GET  /oapi/flow/{flowGuid}?schId=                    → data={flowGuid,name,version,nodeSummary}
+ * - POST /oapi/flow/apply                                → data={instanceId,duplicated}
+ * - GET  /oapi/flow/instance?bizType=&bizId=&schId=      → data={status,version,instanceId,nodeSummary,traceJson,…}
+ * - POST /oapi/flow/callback/reg?schId=                  → data={secret}
+ */
+@RestController
+@RequestMapping("/oapi/flow")
+@RequiredArgsConstructor
+public class OapiFlowController {
+
+    private final OapiFlowService flowService;
+
+    @GetMapping("/list")
+    public List<OapiModels.FlowSummaryVo> list(@RequestParam("schId") Long schId) {
+        return flowService.listFlows(schId);
+    }
+
+    @GetMapping("/{flowGuid}")
+    public OapiModels.FlowSummaryVo getFlow(@PathVariable("flowGuid") String flowGuid,
+                                            @RequestParam("schId") Long schId) {
+        return flowService.getFlow(schId, flowGuid);
+    }
+
+    @PostMapping("/apply")
+    public OapiModels.ApplyResultVo apply(@RequestParam("schId") Long schId,
+                                          @RequestBody OapiModels.ApplyBody body) {
+        return flowService.apply(schId, body);
+    }
+
+    @GetMapping("/instance")
+    public OapiModels.InstanceVo instance(@RequestParam("bizType") String bizType,
+                                          @RequestParam("bizId") String bizId,
+                                          @RequestParam("schId") Long schId) {
+        return flowService.instanceByBiz(schId, bizType, bizId);
+    }
+
+    @PostMapping("/callback/reg")
+    public OapiModels.CallbackRegResultVo registerCallback(
+            @RequestParam("schId") Long schId,
+            @RequestAttribute(value = "yh.app.auth", required = false) String appKey,
+            @RequestBody OapiModels.CallbackRegBody body) {
+        return flowService.registerCallback(schId, appKey, body);
+    }
+}