[CLAUDE] App+Domain+Infra+Api+FE: Notifications module end-to-end

Domain:
- Notification entity + NotificationType enum (stable ints)
- Nullable RefId cho correlation (contract, user, ...)

Infrastructure:
- NotificationConfiguration: bảng Notifications, index theo (UserId, ReadAt)
- NotificationService: ghi vào DbContext, không SaveChanges (để caller quyết
  định unit-of-work — đảm bảo atomic với domain mutation)
- EF migration AddNotifications

Application:
- INotificationService (Notify + NotifyMany)
- CQRS: ListMyNotifications / GetMyUnreadCount / MarkRead / MarkAllRead

Api:
- NotificationsController: GET /api/notifications + unread-count + mark-read

Integration:
- ContractWorkflowService emit notification tới Drafter khi HĐ chuyển phase
  (skip nếu actor chính là Drafter). Title + type theo phase đích:
  DaPhatHanh → ContractPublished, TuChoi → ContractRejected, khác →
  ContractPhaseTransition.

FE:
- Both NotificationBell (admin + user) dùng /api/notifications thật
  (thay cho derived-from-inbox MVP trước đó). 30s refetch, click mark-read,
  'Đọc hết' bulk action.

Foundation sẵn cho SignalR push + email outbox sau này — chỉ cần mở rộng
NotificationService mà không đổi caller.

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

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

@@ -2,15 +2,18 @@ using Microsoft.EntityFrameworkCore;
 using SolutionErp.Application.Common.Exceptions;
 using SolutionErp.Application.Common.Interfaces;
 using SolutionErp.Application.Contracts.Services;
+using SolutionErp.Application.Notifications;
 using SolutionErp.Domain.Contracts;
 using SolutionErp.Domain.Identity;
+using SolutionErp.Domain.Notifications;
 
 namespace SolutionErp.Infrastructure.Services;
 
 public class ContractWorkflowService(
     IApplicationDbContext db,
     IContractCodeGenerator codeGenerator,
-    IDateTime dateTime) : IContractWorkflowService
+    IDateTime dateTime,
+    INotificationService notifications) : IContractWorkflowService
 {
     // Map (from, to) → roles được phép chuyển. Xem docs/workflow-contract.md §5.
     // Admin luôn bypass (check trong Handler trước khi gọi service).
@@ -115,6 +118,31 @@ public class ContractWorkflowService(
             ApprovedAt = dateTime.UtcNow,
         });
 
+        // Notify the drafter (unless they are the actor or contract has no drafter)
+        if (contract.DrafterUserId is Guid drafterId && drafterId != actorUserId)
+        {
+            var title = targetPhase switch
+            {
+                ContractPhase.DaPhatHanh => $"HĐ {contract.MaHopDong ?? contract.TenHopDong} đã phát hành",
+                ContractPhase.TuChoi => $"HĐ {contract.TenHopDong ?? "của bạn"} bị từ chối",
+                _ => $"HĐ {contract.TenHopDong ?? contract.MaHopDong ?? ""} chuyển sang phase mới",
+            };
+            var type = targetPhase switch
+            {
+                ContractPhase.DaPhatHanh => NotificationType.ContractPublished,
+                ContractPhase.TuChoi => NotificationType.ContractRejected,
+                _ => NotificationType.ContractPhaseTransition,
+            };
+            await notifications.NotifyAsync(
+                drafterId,
+                type,
+                title,
+                description: $"{fromPhase} → {targetPhase}" + (string.IsNullOrWhiteSpace(comment) ? "" : $" · {comment}"),
+                href: $"/contracts/{contract.Id}",
+                refId: contract.Id,
+                ct: ct);
+        }
+
         await db.SaveChangesAsync(ct);
     }
 }

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

@@ -0,0 +1,59 @@
+using SolutionErp.Application.Common.Interfaces;
+using SolutionErp.Application.Notifications;
+using SolutionErp.Domain.Notifications;
+
+namespace SolutionErp.Infrastructure.Services;
+
+// MVP: writes directly to DbContext. Does NOT call SaveChanges — caller's unit of
+// work flushes both the domain mutation and the notification atomically.
+// Future: wrap with SignalR IHubContext push + Outbox for email dispatch.
+public class NotificationService(IApplicationDbContext db, IDateTime clock) : INotificationService
+{
+    public Task NotifyAsync(
+        Guid userId,
+        NotificationType type,
+        string title,
+        string? description = null,
+        string? href = null,
+        Guid? refId = null,
+        CancellationToken ct = default)
+    {
+        db.Notifications.Add(new Notification
+        {
+            UserId = userId,
+            Type = type,
+            Title = title,
+            Description = description,
+            Href = href,
+            RefId = refId,
+            CreatedAt = clock.UtcNow,
+        });
+        return Task.CompletedTask;
+    }
+
+    public Task NotifyManyAsync(
+        IEnumerable<Guid> userIds,
+        NotificationType type,
+        string title,
+        string? description = null,
+        string? href = null,
+        Guid? refId = null,
+        CancellationToken ct = default)
+    {
+        var now = clock.UtcNow;
+        foreach (var userId in userIds.Distinct())
+        {
+            db.Notifications.Add(new Notification
+            {
+                UserId = userId,
+                Type = type,
+                Title = title,
+                Description = description,
+                Href = href,
+                RefId = refId,
+                CreatedAt = now,
+            });
+        }
+        return Task.CompletedTask;
+    }
+}