[CLAUDE] Infra: phân cấp phòng ban (Department.ParentId) + /tree + gán phòng cha

Nền cây tổ chức cho trang Hồ sơ Nhân sự (anh: bố trí giống NamGroup; chốt phân
cấp thật + tự gán phòng cha trong quản lý phòng ban).
- Mig AddDepartmentParentId: +ParentId Guid? loose-Guid (no FK vật lý, convention
  PE) + IX_Departments_ParentId. AddColumn+CreateIndex, no new table, Down reversible
  (DropIndex->DropColumn). Chưa apply (CI/prod seed apply).
- GET /api/departments/tree: ráp cây in-memory + đếm NV active theo User.DepartmentId
  (EmployeeProfile KHÔNG có DepartmentId — link qua User, field phòng ban ở User Mig 11)
  + rollup TotalEmployeeCount + cycle-guard HashSet. Authz = class [Authorize] (= GET list).
- Create/Update +ParentId (write vẫn khóa Admin,CatalogManager). Update có cycle-guard:
  chặn tự-làm-cha + đi ngược chuỗi cha gặp lại Id (chống vòng A->B->A).
Build PASS (0 warn/err full solution). Test-after (test-specialist).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Backend/SolutionErp.Infrastructure/Persistence/Configurations/DepartmentConfiguration.cs

@@ -16,6 +16,7 @@ public class DepartmentConfiguration : IEntityTypeConfiguration<Department>
         b.Property(x => x.Note).HasMaxLength(1000);
 
         b.HasIndex(x => x.Code).IsUnique().HasFilter("[IsDeleted] = 0");   // Mig 47 (gotcha #57 EXT) — soft-deleted slot reusable, khớp HasQueryFilter !IsDeleted app-check
+        b.HasIndex(x => x.ParentId);   // cây tổ chức — loose-Guid KHÔNG HasOne self-FK (convention loose-Guid hệ này)
 
         b.HasQueryFilter(x => !x.IsDeleted);
     }

src/Backend/SolutionErp.Infrastructure/Persistence/Migrations/20260616032402_AddDepartmentParentId.cs

@@ -0,0 +1,38 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace SolutionErp.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddDepartmentParentId : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<Guid>(
+                name: "ParentId",
+                table: "Departments",
+                type: "uniqueidentifier",
+                nullable: true);
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Departments_ParentId",
+                table: "Departments",
+                column: "ParentId");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_Departments_ParentId",
+                table: "Departments");
+
+            migrationBuilder.DropColumn(
+                name: "ParentId",
+                table: "Departments");
+        }
+    }
+}

src/Backend/SolutionErp.Infrastructure/Persistence/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -3101,6 +3101,9 @@ namespace SolutionErp.Infrastructure.Persistence.Migrations
                         .HasMaxLength(1000)
                         .HasColumnType("nvarchar(1000)");
 
+                    b.Property<Guid?>("ParentId")
+                        .HasColumnType("uniqueidentifier");
+
                     b.Property<DateTime?>("UpdatedAt")
                         .HasColumnType("datetime2");
 
@@ -3113,6 +3116,8 @@ namespace SolutionErp.Infrastructure.Persistence.Migrations
                         .IsUnique()
                         .HasFilter("[IsDeleted] = 0");
 
+                    b.HasIndex("ParentId");
+
                     b.ToTable("Departments", (string)null);
                 });