Adopt SummerCMS v0.1.1 admin features

Datepickers on native time.Time / lagoon.Date fields, file uploads on
materials, readings and class sessions managed from the course form
(course_id made nullable).

controllers/api/schedule.go

@@ -32,8 +32,11 @@ func bounds(in map[string]any, lower time.Time) (time.Time, *time.Time) {
 	return lower, nil
 }
 
+// preloadSession queries the class sessions students see, with their course
+// and meeting. A session without a course is still pending on an unsaved
+// course form and is left out.
 func preloadSession(q *gorm.DB) *gorm.DB {
-	return q.Preload("Course").Preload("Meeting")
+	return q.Preload("Course").Preload("Meeting").Where("jz_bm_class_sessions.course_id IS NOT NULL")
 }
 
 // Schedule answers GET /schedule?from=YYYY-MM-DD&to=YYYY-MM-DD: the class