feat(server): 工作流扩展 leave 实体骨架（F24）

- workflow_instances 解耦 reimbursement FK：新增 reimbursement_id/leave_request_id + 回填迁移
- 新增 leave_requests 表与 /api/leave-requests 最小链路（create/submit）
- workflow approve/reject/return 支持 leave 状态更新；transfer 非报销任务禁用
- seed-config 增加 leave workflow definition；更新 v3 CSV 状态推进

server/src/app.js

@@ -19,6 +19,7 @@ const meRoutes = require("./routes/me");
 const reimbursementRoutes = require("./routes/reimbursements");
 const invoicePoolRoutes = require("./routes/invoicePool");
 const workflowRoutes = require("./routes/workflow");
+const leaveRoutes = require("./routes/leave");
 const delegationRoutes = require("./routes/delegations");
 const auditRoutes = require("./routes/audit");
 const usersRoutes = require("./routes/users");
@@ -215,6 +216,7 @@ async function buildApp() {
   await app.register(reimbursementRoutes, { prefix: "/api" });
   await app.register(invoicePoolRoutes, { prefix: "/api" });
   await app.register(workflowRoutes, { prefix: "/api" });
+  await app.register(leaveRoutes, { prefix: "/api" });
   await app.register(delegationRoutes, { prefix: "/api" });
   await app.register(auditRoutes, { prefix: "/api" });
   await app.register(usersRoutes, { prefix: "/api" });