[CLAUDE] App+Infra: IChangelogService + log Workflow/Contract/Comment/Attachment

User decision B (log cả 3): mọi thay đổi liên quan HĐ ghi vào unified
ContractChangelogs để render tab Lịch sử FE.

## IChangelogService (Application/Common/Interfaces/)

5 methods:
- LogContractChangeAsync — Header insert/update
- LogDetailChangeAsync — line item insert/update/delete
- LogWorkflowTransitionAsync — phase change (parallel với ContractApprovals)
- LogCommentAddedAsync — góp ý mới
- LogAttachmentAsync — upload/delete file

KHÔNG SaveChanges trong service — caller chịu trách nhiệm save atomic
cùng business changes (pattern giống INotificationService).

## ChangelogService impl

- Resolve actor qua ICurrentUser → UserManager.FindByIdAsync
- Denormalize UserName (FullName ?? Email) cho log readable
- null UserId = system action (vd SLA auto-approve)
- DI: AddScoped trong DependencyInjection.cs

## Wiring vào handlers hiện tại

- ContractWorkflowService.TransitionAsync — LogWorkflowTransitionAsync
  sau khi insert ContractApproval
- CreateContractCommandHandler — LogContractChangeAsync(Insert)
- UpdateContractDraftCommandHandler — diff GiaTri/TenHopDong/NoiDung/
  TemplateId trước update, log update với fieldChangesJson nếu có thay đổi
- AddCommentCommandHandler — LogCommentAddedAsync
- UploadContractAttachmentCommandHandler — LogAttachmentAsync(Insert)
- DeleteContractAttachmentCommandHandler — LogAttachmentAsync(Delete)

Build: dotnet build BE pass (0 error, 2 pre-existing warning trong
DocxRenderer.cs)

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

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

@@ -0,0 +1,44 @@
+using SolutionErp.Domain.Contracts;
+
+namespace SolutionErp.Application.Common.Interfaces;
+
+// Service ghi ContractChangelog. KHÔNG SaveChanges — caller (CQRS handler)
+// chịu trách nhiệm save atomic cùng với business changes. Pattern giống
+// INotificationService.
+public interface IChangelogService
+{
+    Task LogContractChangeAsync(
+        Guid contractId,
+        ChangelogAction action,
+        string? summary = null,
+        string? fieldChangesJson = null,
+        string? contextNote = null,
+        ContractPhase? phaseAtChange = null,
+        CancellationToken ct = default);
+
+    Task LogDetailChangeAsync(
+        Guid contractId,
+        Guid detailId,
+        ChangelogAction action,
+        string? summary = null,
+        string? fieldChangesJson = null,
+        ContractPhase? phaseAtChange = null,
+        CancellationToken ct = default);
+
+    Task LogWorkflowTransitionAsync(
+        Guid contractId,
+        ContractPhase fromPhase,
+        ContractPhase toPhase,
+        string? comment,
+        CancellationToken ct = default);
+
+    Task LogCommentAddedAsync(Guid contractId, string content, ContractPhase phase, CancellationToken ct = default);
+
+    Task LogAttachmentAsync(
+        Guid contractId,
+        Guid attachmentId,
+        ChangelogAction action,
+        string fileName,
+        ContractPhase phase,
+        CancellationToken ct = default);
+}