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/routes/reimbursements.js

@@ -1289,6 +1289,7 @@ async function reimbursementRoutes(fastify) {
             workflow_id: workflowDefinition.id,
             entity_type: "reimbursement",
             entity_id: reimbursementId,
+            reimbursement_id: reimbursementId,
             submitter_id: request.employee.id,
             current_status: "pending",
             current_step_id: actionable.step.id,