[CLAUDE] Domain+Infra: 7 ContractType-specific Details + ContractChangelog (migration 9)

User decision: Option B — bảng riêng cho mỗi loại HĐ (chuẩn nhất, schema
chuyên biệt). Plus: ContractChangelog audit log thống nhất Header /
Detail / Workflow / Comment / Attachment.

## 8 entities mới

### Details (7) — Domain/Contracts/Details/

| Bảng | Loại HĐ | Field đặc trưng |
|---|---|---|
| ThauPhuDetails | 1 (Thầu phụ) | HangMuc, KhoiLuong, DonGia, ThoiGianHoanThanh |
| GiaoKhoanDetails | 2 (Giao khoán) | MaCongViec, KhoiLuong, YeuCauKyThuat |
| NhaCungCapDetails | 3 (NCC) | MaSP, ThongSoKyThuat, SoLuong, ThoiGianGiao, XuatXu |
| DichVuDetails | 4 (Dịch vụ) | MaDichVu, ThoiGian, TuNgay/DenNgay |
| MuaBanDetails | 5 (Mua bán) | MaSP, SoLuong, DonGia, ThueVAT (%), XuatXu |
| NguyenTacNccDetails | 6 (Nguyên tắc NCC) | NhomSP, DonGiaToiThieu/ToiDa, DieuKienGiaoHang |
| NguyenTacDvDetails | 7 (Nguyên tắc DV) | LoaiDichVu, DonGiaToiThieu/ToiDa, PhamViDichVu, SLA |

Common base `ContractDetailBase`: ContractId FK + Order + ThanhTien
decimal(18,2) + GhiChu nvarchar(1000) + audit (BaseEntity).

### ContractChangelog (1) — Domain/Contracts/

Unified audit log. Khác ContractApprovals (workflow-only, dùng cho guard
logic) — Changelog là VIEW LAYER cho user đọc lịch sử thao tác:
- EntityType enum: Contract | Detail | Workflow | Comment | Attachment
- Action enum: Insert | Update | Delete | Transition
- PhaseAtChange snapshot
- UserId + UserName denormalize (log readable)
- Summary human-readable + FieldChangesJson [{Field, Old, New}]
- ContextNote (comment kèm theo)

## EF Configurations

ContractDetailsConfiguration.cs (1 file gộp 7 IEntityTypeConfiguration):
- ToTable + HasMaxLength + HasPrecision per type
- HasOne(Contract).WithMany(<TypeDetails>) cascade delete
- IX (ContractId, Order) cho load timeline

ContractChangelogConfiguration.cs:
- Cascade delete khi Contract xóa
- IX (ContractId, CreatedAt) timeline + IX (ContractId, EntityType) filter

## DbContext + IApplicationDbContext

+ 8 DbSet mới (7 Details + ContractChangelogs).

## Migration 9: AddContractDetailsAndChangelog

3-file rule (gotcha #17): .cs + .Designer.cs + ApplicationDbContextModel
Snapshot.cs đầy đủ. Applied LocalDB SolutionErp_Dev OK — 24 + 8 = 32 bảng
total.

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

src/Backend/SolutionErp.Application/Common/Interfaces/IApplicationDbContext.cs

@@ -1,5 +1,6 @@
 using Microsoft.EntityFrameworkCore;
 using SolutionErp.Domain.Contracts;
+using SolutionErp.Domain.Contracts.Details;
 using SolutionErp.Domain.Forms;
 using SolutionErp.Domain.Identity;
 using SolutionErp.Domain.Master;
@@ -21,11 +22,21 @@ public interface IApplicationDbContext
     DbSet<ContractComment> ContractComments { get; }
     DbSet<ContractAttachment> ContractAttachments { get; }
     DbSet<ContractCodeSequence> ContractCodeSequences { get; }
+    DbSet<ContractChangelog> ContractChangelogs { get; }
     DbSet<Notification> Notifications { get; }
     DbSet<WorkflowTypeAssignment> WorkflowTypeAssignments { get; }
     DbSet<WorkflowDefinition> WorkflowDefinitions { get; }
     DbSet<WorkflowStep> WorkflowSteps { get; }
     DbSet<WorkflowStepApprover> WorkflowStepApprovers { get; }
 
+    // Per-type Details DbSets (Option B — bảng riêng từng loại HĐ)
+    DbSet<ThauPhuDetail> ThauPhuDetails { get; }
+    DbSet<GiaoKhoanDetail> GiaoKhoanDetails { get; }
+    DbSet<NhaCungCapDetail> NhaCungCapDetails { get; }
+    DbSet<DichVuDetail> DichVuDetails { get; }
+    DbSet<MuaBanDetail> MuaBanDetails { get; }
+    DbSet<NguyenTacNccDetail> NguyenTacNccDetails { get; }
+    DbSet<NguyenTacDvDetail> NguyenTacDvDetails { get; }
+
     Task<int> SaveChangesAsync(CancellationToken cancellationToken = default);
 }