[CLAUDE] Office: IT staff tự reassign ticket — authz Admin-OR-IT + capability endpoint (S54)

- BE: GetAssignableItStaffQuery {canReassign,staff} capability endpoint + AssignItTicketHandler authz Admin-OR-dept-IT (Forbidden) + assignee-must-IT (Conflict); controller /assign hạ [Authorize(Roles=Admin)]→[Authorize] (handler enforce fine-grained data-driven)
- FE: fe-admin + fe-user ItTicketsPage SHA256-identical (reverse S53 divergence), nút gate by canReassign, dropdown từ /assignable-staff (không /users)
- Test: +13 authz guard (203→216 PASS), reviewer PASS (role-string Admin chain-verified real)
- No migration (DepartmentId reuse), no menu change

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

src/Backend/SolutionErp.Application/Office/WorkflowAppsFeatures.cs

@@ -440,6 +440,40 @@ public class UpdateItTicketStatusHandler(IApplicationDbContext db, ICurrentUser
     }
 }
 
+// S54: danh sách nhân viên tổ IT có thể nhận ticket + cờ canReassign (Admin HOẶC thành viên dept IT).
+// FE gate nút "Đổi người xử lý" theo CanReassign (BE-computed, tránh silent 403 gotcha #44).
+public record AssignableStaffDto(Guid Id, string FullName);
+public record AssignableStaffResult(bool CanReassign, IReadOnlyList<AssignableStaffDto> Staff);
+public record GetAssignableItStaffQuery : IRequest<AssignableStaffResult>;
+
+public class GetAssignableItStaffHandler(IApplicationDbContext db, ICurrentUser cu)
+    : IRequestHandler<GetAssignableItStaffQuery, AssignableStaffResult>
+{
+    public async Task<AssignableStaffResult> Handle(GetAssignableItStaffQuery q, CancellationToken ct)
+    {
+        if (cu.UserId is null) throw new UnauthorizedException();
+        var itDeptId = await db.Departments.AsNoTracking()
+            .Where(d => d.Code == "IT" && !d.IsDeleted)
+            .Select(d => (Guid?)d.Id)
+            .FirstOrDefaultAsync(ct);
+        var isAdmin = cu.Roles.Contains("Admin");
+        var myDeptId = await db.Users.AsNoTracking()
+            .Where(u => u.Id == cu.UserId.Value)
+            .Select(u => u.DepartmentId)
+            .FirstOrDefaultAsync(ct);
+        var isItStaff = itDeptId is Guid d && myDeptId == d;
+        var canReassign = isAdmin || isItStaff;
+        if (!canReassign || itDeptId is not Guid deptId)
+            return new AssignableStaffResult(canReassign, Array.Empty<AssignableStaffDto>());
+        var staff = await db.Users.AsNoTracking()
+            .Where(u => u.DepartmentId == deptId && u.IsActive)
+            .OrderBy(u => u.FullName)
+            .Select(u => new AssignableStaffDto(u.Id, u.FullName))
+            .ToListAsync(ct);
+        return new AssignableStaffResult(canReassign, staff);
+    }
+}
+
 // P11-D: admin re-assign ticket cho IT staff cụ thể (override round-robin auto-assign
 // lúc Create). Denorm AssignedToFullName từ User.FullName tại thời điểm gán.
 public record AssignItTicketCommand(Guid Id, Guid AssignedToUserId) : IRequest;
@@ -461,9 +495,24 @@ public class AssignItTicketHandler(IApplicationDbContext db, ICurrentUser cu, ID
         if (cu.UserId is null) throw new UnauthorizedException();
         var t = await db.ItTickets.FirstOrDefaultAsync(x => x.Id == req.Id && !x.IsDeleted, ct);
         if (t is null) throw new NotFoundException("ItTicket", req.Id);
+
+        // S54: authz — chỉ Admin HOẶC thành viên tổ IT mới reassign (controller đã hạ [Authorize] any-auth).
+        var itDeptId = await db.Departments.AsNoTracking()
+            .Where(d => d.Code == "IT" && !d.IsDeleted)
+            .Select(d => (Guid?)d.Id)
+            .FirstOrDefaultAsync(ct);
+        var isAdmin = cu.Roles.Contains("Admin");
+        var myDeptId = await db.Users.AsNoTracking()
+            .Where(u => u.Id == cu.UserId.Value).Select(u => u.DepartmentId).FirstOrDefaultAsync(ct);
+        if (!isAdmin && !(itDeptId is Guid mine && myDeptId == mine))
+            throw new ForbiddenException("Chỉ Admin hoặc nhân viên tổ IT mới được gán lại ticket.");
+
         var assignee = await db.Users.AsNoTracking()
             .FirstOrDefaultAsync(u => u.Id == req.AssignedToUserId && u.IsActive, ct);
         if (assignee is null) throw new NotFoundException("User", req.AssignedToUserId);
+        // S54: assignee bắt buộc thuộc tổ IT (khớp dropdown scoped + round-robin).
+        if (!(itDeptId is Guid itd && assignee.DepartmentId == itd))
+            throw new ConflictException("Người được giao phải thuộc tổ IT.");
         t.AssignedToUserId = assignee.Id;
         t.AssignedToFullName = assignee.FullName;
         t.UpdatedAt = clock.UtcNow;