feat: add seating plan drag and quick assessment

LehrerApp.Data.Tests/RepositoryTests.cs

@@ -299,6 +299,38 @@ public sealed class RepositoryTests
         Assert.Throws<InvalidOperationException>(() => new SeatingPlanRepository(db).Save(plan));
     }
 
+    [Fact]
+    public void SeatingPlanRepository_AktualisiertPlanMitFehlendemOptionalenRaum()
+    {
+        using var db = NewInMemoryContext();
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        new GroupRepository(db).Save(group);
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        db.Students.Insert(student);
+        db.Memberships.Insert(new GroupMembership { GroupId = group.Id, StudentId = student.Id });
+        var plan = new SeatingPlan
+        {
+            GroupId = group.Id,
+            Name = "Standard",
+            Room = null!,
+            Rows = 2,
+            Columns = 2,
+        };
+        db.SeatingPlans.Insert(plan);
+        plan.Assignments.Add(new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id });
+
+        new SeatingPlanRepository(db).Save(plan);
+
+        var saved = db.SeatingPlans.FindById(plan.Id);
+        Assert.NotNull(saved);
+        Assert.True(string.IsNullOrEmpty(saved.Room));
+        Assert.Equal(student.Id, saved.Assignments.Single().StudentId);
+
+        // LiteDB may materialize the empty optional value as null again. A later
+        // drag/drop save must therefore remain safe as well.
+        new SeatingPlanRepository(db).Save(saved);
+    }
+
     [Fact]
     public void GroupMembershipRepository_Save_LehntZweiteZuordnungFuerGleichesPaarAb()
     {