[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.Infrastructure/DependencyInjection.cs

@@ -34,6 +34,7 @@ public static class DependencyInjection
         services.AddScoped<IContractWorkflowService, ContractWorkflowService>();
         services.AddScoped<IContractExcelExporter, ContractExcelExporter>();
         services.AddScoped<INotificationService, NotificationService>();
+        services.AddScoped<IChangelogService, ChangelogService>();
         services.AddSingleton<IFileStorage, LocalFileStorage>();
 
         // Phase 3 iteration 2 — SLA auto-approve background service

src/Backend/SolutionErp.Infrastructure/Services/ChangelogService.cs

@@ -0,0 +1,132 @@
+using Microsoft.AspNetCore.Identity;
+using SolutionErp.Application.Common.Interfaces;
+using SolutionErp.Domain.Contracts;
+using SolutionErp.Domain.Identity;
+
+namespace SolutionErp.Infrastructure.Services;
+
+// Impl IChangelogService. Resolve current user qua ICurrentUser (null = system,
+// vd SLA auto-approve). Denormalize UserName để log readable mà không cần JOIN
+// Users table khi render.
+public class ChangelogService : IChangelogService
+{
+    private readonly IApplicationDbContext _db;
+    private readonly ICurrentUser _currentUser;
+    private readonly UserManager<User> _userManager;
+
+    public ChangelogService(IApplicationDbContext db, ICurrentUser currentUser, UserManager<User> userManager)
+    {
+        _db = db;
+        _currentUser = currentUser;
+        _userManager = userManager;
+    }
+
+    private async Task<(Guid? Id, string? Name)> ResolveActorAsync()
+    {
+        if (_currentUser.UserId is null) return (null, null);
+        var user = await _userManager.FindByIdAsync(_currentUser.UserId.Value.ToString());
+        return (user?.Id, user?.FullName ?? user?.Email);
+    }
+
+    public async Task LogContractChangeAsync(
+        Guid contractId, ChangelogAction action, string? summary = null,
+        string? fieldChangesJson = null, string? contextNote = null,
+        ContractPhase? phaseAtChange = null, CancellationToken ct = default)
+    {
+        var (uid, uname) = await ResolveActorAsync();
+        _db.ContractChangelogs.Add(new ContractChangelog
+        {
+            ContractId = contractId,
+            EntityType = ChangelogEntityType.Contract,
+            EntityId = null,
+            Action = action,
+            PhaseAtChange = phaseAtChange,
+            UserId = uid,
+            UserName = uname,
+            Summary = summary,
+            FieldChangesJson = fieldChangesJson,
+            ContextNote = contextNote,
+        });
+    }
+
+    public async Task LogDetailChangeAsync(
+        Guid contractId, Guid detailId, ChangelogAction action,
+        string? summary = null, string? fieldChangesJson = null,
+        ContractPhase? phaseAtChange = null, CancellationToken ct = default)
+    {
+        var (uid, uname) = await ResolveActorAsync();
+        _db.ContractChangelogs.Add(new ContractChangelog
+        {
+            ContractId = contractId,
+            EntityType = ChangelogEntityType.Detail,
+            EntityId = detailId,
+            Action = action,
+            PhaseAtChange = phaseAtChange,
+            UserId = uid,
+            UserName = uname,
+            Summary = summary,
+            FieldChangesJson = fieldChangesJson,
+        });
+    }
+
+    public async Task LogWorkflowTransitionAsync(
+        Guid contractId, ContractPhase fromPhase, ContractPhase toPhase,
+        string? comment, CancellationToken ct = default)
+    {
+        var (uid, uname) = await ResolveActorAsync();
+        _db.ContractChangelogs.Add(new ContractChangelog
+        {
+            ContractId = contractId,
+            EntityType = ChangelogEntityType.Workflow,
+            EntityId = null,
+            Action = ChangelogAction.Transition,
+            PhaseAtChange = toPhase,
+            UserId = uid,
+            UserName = uname ?? "Hệ thống",
+            Summary = $"Chuyển phase {fromPhase} → {toPhase}",
+            ContextNote = comment,
+        });
+    }
+
+    public async Task LogCommentAddedAsync(
+        Guid contractId, string content, ContractPhase phase, CancellationToken ct = default)
+    {
+        var (uid, uname) = await ResolveActorAsync();
+        _db.ContractChangelogs.Add(new ContractChangelog
+        {
+            ContractId = contractId,
+            EntityType = ChangelogEntityType.Comment,
+            EntityId = null,
+            Action = ChangelogAction.Insert,
+            PhaseAtChange = phase,
+            UserId = uid,
+            UserName = uname,
+            Summary = "Thêm góp ý",
+            ContextNote = content.Length > 200 ? content[..200] + "..." : content,
+        });
+    }
+
+    public async Task LogAttachmentAsync(
+        Guid contractId, Guid attachmentId, ChangelogAction action,
+        string fileName, ContractPhase phase, CancellationToken ct = default)
+    {
+        var (uid, uname) = await ResolveActorAsync();
+        var verb = action switch
+        {
+            ChangelogAction.Insert => "Tải lên",
+            ChangelogAction.Delete => "Xóa",
+            _ => "Cập nhật",
+        };
+        _db.ContractChangelogs.Add(new ContractChangelog
+        {
+            ContractId = contractId,
+            EntityType = ChangelogEntityType.Attachment,
+            EntityId = attachmentId,
+            Action = action,
+            PhaseAtChange = phase,
+            UserId = uid,
+            UserName = uname,
+            Summary = $"{verb} file: {fileName}",
+        });
+    }
+}

src/Backend/SolutionErp.Infrastructure/Services/ContractWorkflowService.cs

@@ -16,7 +16,8 @@ public class ContractWorkflowService(
     IApplicationDbContext db,
     IContractCodeGenerator codeGenerator,
     IDateTime dateTime,
-    INotificationService notifications) : IContractWorkflowService
+    INotificationService notifications,
+    IChangelogService changelog) : IContractWorkflowService
 {
     // Expose per-policy SLA via the contract — accepts optional contract so the
     // caller (CreateContractCommand) can ask for a specific type's SLA even
@@ -101,6 +102,9 @@ public class ContractWorkflowService(
             ApprovedAt = dateTime.UtcNow,
         });
 
+        // Log workflow transition vào unified Changelog (cho user xem trên tab Lịch sử)
+        await changelog.LogWorkflowTransitionAsync(contract.Id, fromPhase, targetPhase, comment);
+
         if (contract.DrafterUserId is Guid drafterId && drafterId != actorUserId)
         {
             var title = targetPhase switch