feat: zjazd (meeting) on readings and materials

The Pulpit hero lists what to read for the current or next zjazd, so
readings and materials get a nullable meeting_id (FK to jz_bm_meetings,
ON DELETE SET NULL, indexed). The reading admin form gets a "Na zjazd"
picker, the student reading JSON carries meeting_id, and the management
/readings records filter and validate it.

models/reading.go

@@ -13,6 +13,8 @@ type Reading struct {
 	ID         uint      `gorm:"column:id;primaryKey"`
 	CourseID   *uint     `gorm:"column:course_id"`
 	Course     *Course   `gorm:"foreignKey:CourseID"`
+	MeetingID  *uint     `gorm:"column:meeting_id"`
+	Meeting    *Meeting  `gorm:"foreignKey:MeetingID"`
 	Title      string    `gorm:"column:title"`
 	Author     *string   `gorm:"column:author"`
 	Source     *string   `gorm:"column:source"`
@@ -28,7 +30,7 @@ func (Reading) TableName() string { return "jz_bm_readings" }
 
 // Fillable lists the columns mass assignment may set.
 func (Reading) Fillable() []string {
-	return []string{"course_id", "title", "author", "source", "url", "is_required", "sort_order"}
+	return []string{"course_id", "meeting_id", "title", "author", "source", "url", "is_required", "sort_order"}
 }
 
 // Rules are checked by the admin API on every save.

models/readings/fields.yaml

@@ -31,3 +31,10 @@ fields:
         type: number
         span: left
         default: 0
+    meeting:
+        label: jz.bm::lang.common.meeting
+        type: relation
+        nameFrom: number
+        span: right
+        emptyOption: jz.bm::lang.common.none
+        comment: jz.bm::lang.common.meeting_comment

models/readings/fields_course.yaml

@@ -1,5 +1,6 @@
 # The reading form inside the course form's relation manager: the course is
-# the parent, so there is no course picker.
+# the parent, so there is no course picker, and a relation form has no
+# relation pickers, so the meeting (zjazd) is set on the readings screen.
 fields:
     title:
         label: jz.bm::lang.common.title