[CLAUDE] Drastic refactor: flat workflow Phòng × Cấp + Migration 21 (Chunk A)

User chốt drastic refactor — bỏ phase enum hoàn toàn, dùng ChoDuyet=10
đơn nhất + currentStepIndex tracking. Workflow flat list (Phòng × Cấp ×
Approvers). Mỗi PE/HĐ pin WorkflowDefinitionId chạy hết quy trình đó.

Schema (Migration 21 `RefactorWorkflowToFlatModel`):
- Phase enum +ChoDuyet=10 (PE + Contract). Legacy 2-9 + 98 deprecated.
- WorkflowStep + DepartmentId Guid? (FK Restrict) + PositionLevel int?
  (PE + Contract — mirror).
- PE/Contract + CurrentWorkflowStepIndex int? + RejectedAtStepIndex int?
- DROP table PurchaseEvaluationWorkflowStepInnerSteps (Mig 18)
- DROP table WorkflowStepInnerSteps (Mig 20)
- DROP column ContractDeptApproval.InnerStepId (Mig 20)
- DROP column PEDeptApproval.InnerStepId (Mig 18)
- DROP filtered indexes (Mig 19/20) + restore simple unique
  (TargetId, Phase, Dept, Stage) non-filtered

Service rewrite (PE + Contract WorkflowService.TransitionAsync):
- Phase transitions: DangSoanThao → ChoDuyet (Drafter trình, init idx=0)
- ChoDuyet → ChoDuyet (advance idx per approve)
- ChoDuyet → DaDuyet/DaPhatHanh (idx >= steps.Count → terminal)
- ChoDuyet → DangSoanThao (Trả lại — save RejectedAtStepIndex)
- ChoDuyet → TuChoi (Từ chối — khoá vĩnh viễn)
- DangSoanThao + RejectedAtStepIndex → ChoDuyet jump-back to saved idx
- Approver match: actor.Dept == step.Dept AND actor.PositionLevel >=
  step.PositionLevel (OR-of-many cùng cấp/dept = pass) OR
  Approvers.Any(Kind=User AND id match) OR
  Approvers.Any(Kind=Role AND actorRoles contains)
- Admin role bypass policy. Last step done → gen mã HĐ (Contract only)

App CQRS:
- WorkflowStepDto + WorkflowStepInput drop InnerStep, add DepartmentId
  + PositionLevel fields. PE + Contract mirror.

Tests rewrite:
- DROP PeNStageApprovalTests.cs (6 test) + ContractNStageApprovalTests.cs
  (6 test) + PeTwoStageApprovalTests.cs (7 test) — legacy N-stage/2-stage
  no longer applicable
- UPDATE PeWorkflowAdminTests signature to new flat input
- 96 → 77 test pass (drop 19 legacy)

Reference Domain entities removed:
- WorkflowStepInnerStep (Contract)
- PurchaseEvaluationWorkflowStepInnerStep (PE)
- DTOs WorkflowStepInnerStepDto / CreateWorkflowStepInnerStepInput per module

Memory `feedback_drastic_refactor_scope.md` validated: drastic refactor
done in dedicated session với context fresh, scope ~5h actual (planned ~8-10h
with 2x buffer).

Verify:
- dotnet build SolutionErp.slnx 0 error
- dotnet ef database update Mig 21 LocalDB applied OK
- dotnet test 77 pass (54 Domain + 23 Infra)
- 3-file rule: Migration .cs + Designer.cs + Snapshot updated

Pending Chunk B: FE Designer flat UI (PeWorkflowsPage + WorkflowsPage).
Pending Chunk C: FE PeWorkflowPanel + workflow timeline display.
Pending Chunk D: Docs + Skill + Memory + session log.

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

tests/SolutionErp.Infrastructure.Tests/Application/PeWorkflowAdminTests.cs

@@ -23,12 +23,14 @@ public class CreatePeWorkflowDefinitionCommandHandlerTests
             Description: null,
             Steps: new List<CreatePeWorkflowStepInput>
             {
-                new(Order: 1, Phase: (int)PurchaseEvaluationPhase.DangSoanThao, Name: "Soạn", SlaDays: 3,
+                new(Order: 1, Phase: (int)PurchaseEvaluationPhase.ChoDuyet, Name: "Soạn",
+                    SlaDays: 3, DepartmentId: null, PositionLevel: null,
                     Approvers: new List<CreatePeWorkflowStepApproverInput>
                     {
                         new(Kind: (int)WorkflowApproverKind.Role, AssignmentValue: AppRoles.Drafter),
                     }),
-                new(Order: 2, Phase: (int)PurchaseEvaluationPhase.ChoCCM, Name: "CCM duyệt", SlaDays: 2,
+                new(Order: 2, Phase: (int)PurchaseEvaluationPhase.ChoDuyet, Name: "CCM duyệt",
+                    SlaDays: 2, DepartmentId: null, PositionLevel: null,
                     Approvers: new List<CreatePeWorkflowStepApproverInput>
                     {
                         new(Kind: (int)WorkflowApproverKind.Role, AssignmentValue: AppRoles.CostControl),
@@ -110,10 +112,10 @@ public class CreatePeWorkflowDefinitionCommandHandlerTests
 
         steps.Should().HaveCount(2);
         steps[0].Order.Should().Be(1);
-        steps[0].Phase.Should().Be(PurchaseEvaluationPhase.DangSoanThao);
+        steps[0].Phase.Should().Be(PurchaseEvaluationPhase.ChoDuyet);
         steps[0].Name.Should().Be("Soạn");
         steps[1].Order.Should().Be(2);
-        steps[1].Phase.Should().Be(PurchaseEvaluationPhase.ChoCCM);
+        steps[1].Phase.Should().Be(PurchaseEvaluationPhase.ChoDuyet);
     }
 
     [Fact]