fix: retain cultivate feed media references

server/internal_cultivate_test.go

@@ -46,3 +46,17 @@ func TestInternalCultivateUsesSharedInternalCredential(t *testing.T) {
 		t.Fatalf("shared internal credential was rejected: status=%d body=%s", rec.Code, rec.Body.String())
 	}
 }
+
+func TestCultivateFileReferenceTopic(t *testing.T) {
+	first := cultivateFileReferenceTopic(1577072518975639552, 0)
+	second := cultivateFileReferenceTopic(1577072518975639552, 1)
+	if first == second {
+		t.Fatalf("file references must be unique per media item: %q", first)
+	}
+	if len(first) > 25 || len(second) > 25 {
+		t.Fatalf("file reference exceeds im_filemsglinks.topic capacity: %q %q", first, second)
+	}
+	if first != cultivateFileReferenceTopic(1577072518975639552, 0) {
+		t.Fatalf("file reference is not deterministic: %q", first)
+	}
+}