feat(server): 站内通知基础设施与关键事件触发(F14,F30)

- 新增 notifications 表与 API(list/mark-read)
- submit/approve/reject/return/transfer/pay/forgot-password 写入站内通知
- 通知失败降级，不阻塞主流程

server/prisma/schema.prisma

@@ -172,6 +172,7 @@ model Employee {
 
   auth_sessions AuthSession[]
   audit_logs    AuditLog[]    @relation("AuditActor")
+  notifications Notification[]
 
   submitted_reimbursements ReimbursementForm[] @relation("ReimbursementSubmitter")
   owned_reimbursements     ReimbursementForm[] @relation("ReimbursementOwner")
@@ -286,6 +287,24 @@ model AuditLog {
   @@map("audit_logs")
 }
 
+model Notification {
+  id                    String   @id @default(uuid()) @db.Uuid
+  recipient_employee_id String   @db.Uuid
+  type                  String   @db.VarChar(64)
+  title                 String   @db.VarChar(200)
+  body                  String?
+  target_type           String?  @db.VarChar(64)
+  target_id             String?  @db.Uuid
+  read_at               DateTime?
+  created_at            DateTime @default(now())
+
+  recipient Employee @relation(fields: [recipient_employee_id], references: [id], onDelete: Cascade)
+
+  @@index([recipient_employee_id, created_at])
+  @@index([recipient_employee_id, read_at, created_at], map: "idx_notifications_unread_query")
+  @@map("notifications")
+}
+
 model ExpenseType {
   id                   String          @id @default(uuid()) @db.Uuid
   code                 String          @unique