improvement: task assignment flow added. cases and tasks feature updated accordingly.

backend/prisma/schema.prisma

@@ -25,6 +25,7 @@ model User {
   sentStaffInvites   StaffInvitation[]
   sentOrganizationInvitations OrganizationInvitation[]
   statusChangedLabCaseTasks LabCaseTask[] @relation("LabCaseTaskLastStatusChangedBy")
+  assignedLabCaseTasks LabCaseTask[] @relation("LabCaseTaskAssignee")
   labCaseTaskStatusEvents LabCaseTaskStatusEvent[]
   labCaseComments LabCaseComment[]
   phoneVerificationCodes PhoneVerificationCode[]
@@ -353,11 +354,14 @@ model LabCaseTask {
   stepOrder          Int
   stepLabel          String
   status             LabTaskStatus @default(IN_PROGRESS)
+  assigneeUserId     String?
+  assignedAt         DateTime?
   lastStatusChangedByUserId String?
   lastStatusChangedAt       DateTime?
 
   labCase  LabCase         @relation(fields: [labCaseId], references: [id], onDelete: Cascade)
   detail   TreatmentDetail @relation(fields: [treatmentDetailId], references: [id], onDelete: Cascade)
+  assignee User?           @relation("LabCaseTaskAssignee", fields: [assigneeUserId], references: [id], onDelete: SetNull)
   lastStatusChangedBy User? @relation("LabCaseTaskLastStatusChangedBy", fields: [lastStatusChangedByUserId], references: [id], onDelete: SetNull)
   statusEvents LabCaseTaskStatusEvent[]
 
@@ -366,6 +370,7 @@ model LabCaseTask {
 
   @@unique([labCaseId, treatmentDetailId, prosthesisTypeCode, stepOrder])
   @@index([labCaseId, status])
+  @@index([assigneeUserId])
   @@map("lab_case_tasks")
 }