feat: lecturer on class sessions and in the session course ref

The schedule showed a lecturer only where the session notes named one
(Globalizacja), so the student plan had nobody under the other courses.
The session JSON now carries the course lecturer in its course ref and an
optional per-session lecturer (new nullable column, admin field, seed key)
for courses shared between two people. The student app shows the
session's own lecturer or, failing that, the course's.

plugin_test.go

@@ -77,8 +77,8 @@ func TestPluginActivates(t *testing.T) {
 	if n := len(p.Models()); n != 10 {
 		t.Errorf("Models = %d, want 10", n)
 	}
-	if n := len(p.Migrations()); n != 12 {
-		t.Errorf("Migrations = %d, want 12", n)
+	if n := len(p.Migrations()); n != 13 {
+		t.Errorf("Migrations = %d, want 13", n)
 	}
 	if !slices.ContainsFunc(p.Commands(), func(c bonfire.Command) bool { return c.Name == "bm:seed" }) {
 		t.Error("bm:seed is not registered")