[CLAUDE] Domain+Infra: Migration 20 Contract workflow inner steps mirror PE (Chunk A)

Mirror PE N-stage Mig 18+19 pattern sang Contract module. Gộp 1 migration
(CREATE TABLE + ALTER + filtered unique alter) thay vì tách 2 như PE.

Schema:
- entity WorkflowStepInnerStep (Domain/Contracts/) — Order, DeptId,
  PositionLevel, Name, SlaDays, IsRequired
- WorkflowStep nav +InnerSteps List
- ContractDepartmentApproval +InnerStepId Guid? FK Restrict
- CREATE TABLE WorkflowStepInnerSteps (no Contract prefix vì module dùng
  entity gốc Workflow* từ Mig 8)
- DROP UX_ContractDeptApprovals_Contract_Phase_Dept_Stage cũ
- RECREATE filtered: WHERE InnerStepId IS NULL (legacy 2-stage Mig 16)
  + new filtered UNIQUE (ContractId, Phase, InnerStepId) WHERE InnerStepId
  IS NOT NULL (N-stage)
- 3 INDEX: IX_InnerStepId, IX_(StepId, Order), IX_DeptId

Backward compat 100%: workflow Contract no InnerSteps configured →
service fallback legacy 2-stage. Data legacy InnerStepId=null vẫn enforce
unique cũ qua filtered index.

Note: Budget defer (chưa có versioned WorkflowDefinition entity —
hardcoded BudgetPolicy.Default). Cần migration AddBudgetVersionedWorkflow
trước khi mirror N-stage Budget.

Verify:
- dotnet build SolutionErp.slnx 0 error
- dotnet ef database update LocalDB applied OK
- dotnet test 89 pass (54 + 35) — no regression

Pending Chunk B: WorkflowAdminFeatures.cs DTO/Input/Validator/Handler
extend mirror PE Chunk B pattern.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Backend/SolutionErp.Domain/Contracts/ContractDepartmentApproval.cs

@@ -23,5 +23,9 @@ public class ContractDepartmentApproval : AuditableEntity
     public DateTime ApprovedAt { get; set; }
     public bool IsBypassed { get; set; }                    // true nếu NV bypass (User.CanBypassReview=true)
 
+    // N-stage inner step link (Mig 20) — null cho data legacy 2-stage Review/Confirm.
+    // Có giá trị khi step cha có InnerSteps configured. Mirror PE Mig 18 pattern.
+    public Guid? InnerStepId { get; set; }
+
     public Contract? Contract { get; set; }
 }

src/Backend/SolutionErp.Domain/Contracts/WorkflowDefinition.cs

@@ -1,4 +1,5 @@
 using SolutionErp.Domain.Common;
+using SolutionErp.Domain.Identity;     // reuse PositionLevel
 
 namespace SolutionErp.Domain.Contracts;
 
@@ -32,6 +33,11 @@ public class WorkflowStep : BaseEntity
 
     public WorkflowDefinition? WorkflowDefinition { get; set; }
     public List<WorkflowStepApprover> Approvers { get; set; } = new();
+
+    // Inner steps (Mig 20) — N-stage approval Phòng × PositionLevel sequential.
+    // Mirror PE pattern (Mig 18). Empty list → service fallback logic 2-stage
+    // Review/Confirm legacy (Mig 16) per dept.
+    public List<WorkflowStepInnerStep> InnerSteps { get; set; } = new();
 }
 
 public enum WorkflowApproverKind
@@ -48,3 +54,23 @@ public class WorkflowStepApprover : BaseEntity
 
     public WorkflowStep? Step { get; set; }
 }
+
+// Inner step (Mig 20 — Phase 9+) — sub-step level con cấu hình bên trong 1
+// WorkflowStep cha (= 1 phase). Mirror PurchaseEvaluationWorkflowStepInnerStep
+// pattern (Mig 18). Cho phép admin định nghĩa thứ tự duyệt N-stage theo
+// Department × PositionLevel: NV.A → PP.A → TP.A → NV.B → PP.B → TP.B → ...
+//
+// User khớp DepartmentId + PositionLevel + Order tiếp theo chưa duyệt = approver
+// hợp lệ. CanBypassReview ở User cho TP skip NV+PP cùng dept (audit IsBypassed).
+public class WorkflowStepInnerStep : BaseEntity
+{
+    public Guid WorkflowStepId { get; set; }
+    public int Order { get; set; }
+    public Guid DepartmentId { get; set; }
+    public PositionLevel PositionLevel { get; set; }            // NV / PP / TP
+    public string? Name { get; set; }                            // hiển thị FE — vd "NV.PRO duyệt"
+    public int? SlaDays { get; set; }
+    public bool IsRequired { get; set; } = true;
+
+    public WorkflowStep? Step { get; set; }
+}