Enhance staffing management with uncovered teacher tracking

- Updated `protocol.md` to clarify the concept of uncovered subjects and the number of additional teachers required.
- Introduced `teachersShort` property in the `StaffingSubject` interface to indicate how many more teachers are needed for a subject.
- Enhanced localization strings to reflect the new uncovered teacher information in both English and Russian.
- Updated the management panel UI to display the number of teachers short for each uncovered subject.
- Revised the `Uncovered` method in the staffing logic to return detailed information about uncovered subjects, including the shortfall of teachers.
- Added tests to validate the new uncovered teacher tracking functionality and ensure accurate reporting in the staffing API.
- Marked related tasks as complete in the documentation for the golden fixtures phase.

tests/HSchool.AppHost.Tests/StaffingApiTests.cs

@@ -32,7 +32,8 @@ public class StaffingApiTests(AppHostFixture fixture)
         Assert.Equal(32, staffing.Applicants.Count);
         Assert.Empty(staffing.Staff);
         Assert.Contains(staffing.Uncovered, subject => subject.DefName == "Mathematics");
-        Assert.Contains(staffing.Uncovered, subject => subject.DefName == "PrimarySchool");
+        var primary = Assert.Single(staffing.Uncovered, subject => subject.DefName == "PrimarySchool");
+        Assert.Equal(3, primary.TeachersShort);
         Assert.NotEmpty(staffing.Positions);
         Assert.Contains(staffing.Positions, position => position.DefName == "Teacher");
         Assert.Contains(staffing.Subjects, subject => subject.DefName == "Mathematics");
@@ -331,7 +332,7 @@ public class StaffingApiTests(AppHostFixture fixture)
         IReadOnlyList<DefLabelResponse> Positions,
         IReadOnlyList<UncoveredSubjectResponse> Subjects);
 
-    private sealed record UncoveredSubjectResponse(string DefName, string Label, int GradeMin, int GradeMax, int HoursPerWeek);
+    private sealed record UncoveredSubjectResponse(string DefName, string Label, int GradeMin, int GradeMax, int HoursPerWeek, int TeachersShort);
 
     private sealed record ApplicantResponse(
         string Id,