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/prisma/schema.prisma

@@ -36,6 +36,14 @@ enum ReimbursementStatus {
   cancelled
 }
 
+enum LeaveRequestStatus {
+  draft
+  reviewing
+  approved
+  rejected
+  cancelled
+}
+
 enum InvoicePoolStatus {
   pending_ocr
   ocr_failed
@@ -126,6 +134,7 @@ model Department {
 
   employees           Employee[]
   reimbursement_forms ReimbursementForm[]
+  leave_requests      LeaveRequest[]
   workflow_instances  WorkflowInstance[]
 
   @@index([parent_id])
@@ -177,6 +186,7 @@ model Employee {
   submitted_reimbursements ReimbursementForm[] @relation("ReimbursementSubmitter")
   owned_reimbursements     ReimbursementForm[] @relation("ReimbursementOwner")
   paid_reimbursements      ReimbursementForm[] @relation("ReimbursementPaidBy")
+  leave_requests           LeaveRequest[]
 
   owned_invoices      InvoicePool[] @relation("InvoiceOwner")
   uploaded_invoices   InvoicePool[] @relation("InvoiceUploader")
@@ -408,6 +418,32 @@ model ReimbursementItem {
   @@map("reimbursement_items")
 }
 
+model LeaveRequest {
+  id            String             @id @default(uuid()) @db.Uuid
+  owner_id      String             @db.Uuid
+  department_id String             @db.Uuid
+  start_date    DateTime           @db.Date
+  end_date      DateTime           @db.Date
+  reason        String?
+  status        LeaveRequestStatus @default(draft)
+
+  created_at DateTime @default(now())
+  created_by String?  @db.Uuid
+  updated_at DateTime @updatedAt
+  updated_by String?  @db.Uuid
+
+  owner      Employee   @relation(fields: [owner_id], references: [id], onDelete: Restrict)
+  department Department @relation(fields: [department_id], references: [id], onDelete: Restrict)
+
+  workflow_instances WorkflowInstance[]
+
+  @@index([owner_id])
+  @@index([department_id])
+  @@index([status])
+  @@index([owner_id, status, updated_at], map: "idx_leave_owner_status_updated_at")
+  @@map("leave_requests")
+}
+
 model OcrResult {
   id           String      @id @default(uuid()) @db.Uuid
   file_sha256  String
@@ -513,6 +549,8 @@ model WorkflowInstance {
   workflow_id     String                 @db.Uuid
   entity_type     String
   entity_id       String                 @db.Uuid
+  reimbursement_id String?               @db.Uuid
+  leave_request_id String?               @db.Uuid
   submitter_id    String                 @db.Uuid
   current_status  WorkflowInstanceStatus @default(pending)
   current_step_id String
@@ -526,7 +564,8 @@ model WorkflowInstance {
   updated_by String?  @db.Uuid
 
   workflow_definition WorkflowDefinition @relation(fields: [workflow_id], references: [id], onDelete: Restrict)
-  reimbursement_form  ReimbursementForm  @relation(fields: [entity_id], references: [id], onDelete: Cascade)
+  reimbursement_form  ReimbursementForm? @relation(fields: [reimbursement_id], references: [id], onDelete: Cascade)
+  leave_request       LeaveRequest?      @relation(fields: [leave_request_id], references: [id], onDelete: Cascade)
   submitter           Employee           @relation("WorkflowInstanceSubmitter", fields: [submitter_id], references: [id], onDelete: Restrict)
   department          Department?        @relation(fields: [context_department_id], references: [id], onDelete: SetNull)
 
@@ -538,6 +577,8 @@ model WorkflowInstance {
 
   @@index([workflow_id])
   @@index([entity_type, entity_id])
+  @@index([reimbursement_id])
+  @@index([leave_request_id])
   @@index([current_status])
   @@index([submitter_id])
   @@index([context_department_id])