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.

controllers/api/json_test.go

@@ -0,0 +1,28 @@
+package api
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+// TestReadingJSONCarriesMeetingID checks that meeting_id is always present
+// in a reading, as a number or as null.
+func TestReadingJSONCarriesMeetingID(t *testing.T) {
+	id := uint(4)
+	for _, tc := range []struct {
+		meeting *uint
+		want    string
+	}{
+		{&id, `"meeting_id":4`},
+		{nil, `"meeting_id":null`},
+	} {
+		b, err := json.Marshal(readingJSON{ID: 1, Title: "Lektura", MeetingID: tc.meeting})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !strings.Contains(string(b), tc.want) {
+			t.Errorf("reading JSON %s does not contain %s", b, tc.want)
+		}
+	}
+}