Callbacks service docs updated

api/pkg/db/internal/mongo/callbacksdb/callback.go

@@ -0,0 +1,8 @@
+package callbacksdb
+
+import "github.com/tech/sendico/pkg/model"
+
+type callbackInternal struct {
+	model.Callback `bson:",inline" json:",inline"`
+	SecretRef      string `bson:"secret_ref" json:"-"`
+}