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.

controllers/api/json.go

@@ -17,6 +17,7 @@ type courseRef struct {
 	Code     *string `json:"code"`
 	Name     string  `json:"name"`
 	Form     string  `json:"form"`
+	Lecturer *string `json:"lecturer"`
 	Semester int     `json:"semester"`
 }
 
@@ -24,7 +25,7 @@ func newCourseRef(c *models.Course) *courseRef {
 	if c == nil {
 		return nil
 	}
-	return &courseRef{ID: c.ID, Code: c.Code, Name: c.Name, Form: c.Form, Semester: c.Semester}
+	return &courseRef{ID: c.ID, Code: c.Code, Name: c.Name, Form: c.Form, Lecturer: c.Lecturer, Semester: c.Semester}
 }
 
 // courseJSON is a course's own fields.
@@ -105,6 +106,7 @@ type sessionJSON struct {
 	EndsAt        string     `json:"ends_at"`
 	Room          *string    `json:"room"`
 	Kind          *string    `json:"kind"`
+	Lecturer      *string    `json:"lecturer"`
 	Notes         *string    `json:"notes"`
 	IsOnline      bool       `json:"is_online"`
 	Course        *courseRef `json:"course"`
@@ -119,6 +121,7 @@ func newSessionJSON(s models.ClassSession) sessionJSON {
 		EndsAt:    models.FormatDateTime(s.EndsAt),
 		Room:      s.Room,
 		Kind:      s.Kind,
+		Lecturer:  s.Lecturer,
 		Notes:     s.Notes,
 		IsOnline:  s.IsOnline,
 		Course:    newCourseRef(s.Course),