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.

updates/20261027120000_add_meeting_to_readings_and_materials.go

@@ -0,0 +1,29 @@
+package updates
+
+import (
+	"github.com/go-gormigrate/gormigrate/v2"
+	"gorm.io/gorm"
+)
+
+// AddMeetingToReadingsAndMaterials returns the
+// 20261027120000_add_meeting_to_readings_and_materials gormigrate entry: a
+// reading or a material may be assigned to the zjazd (meeting) it should be
+// read for, so the dashboard can suggest it before that zjazd. Deleting the
+// meeting keeps the rows and clears the assignment.
+func AddMeetingToReadingsAndMaterials() *gormigrate.Migration {
+	return &gormigrate.Migration{
+		ID: "20261027120000_add_meeting_to_readings_and_materials",
+		Migrate: func(tx *gorm.DB) error {
+			return execAll(tx, `
+ALTER TABLE jz_bm_readings ADD COLUMN meeting_id BIGINT NULL REFERENCES jz_bm_meetings (id) ON DELETE SET NULL;
+CREATE INDEX jz_bm_readings_meeting_id_index ON jz_bm_readings (meeting_id);
+ALTER TABLE jz_bm_materials ADD COLUMN meeting_id BIGINT NULL REFERENCES jz_bm_meetings (id) ON DELETE SET NULL;
+CREATE INDEX jz_bm_materials_meeting_id_index ON jz_bm_materials (meeting_id)`)
+		},
+		Rollback: func(tx *gorm.DB) error {
+			return execAll(tx, `
+ALTER TABLE jz_bm_readings DROP COLUMN IF EXISTS meeting_id;
+ALTER TABLE jz_bm_materials DROP COLUMN IF EXISTS meeting_id`)
+		},
+	}
+}