Add series data structure

lib/DB/Series.rakumod

@@ -0,0 +1,15 @@
+use v6.e.PREVIEW;
+
+use JSON::Class:auth<zef:vrurg>;
+
+#| A plain markdown post
+unit class Series is json(:pretty);
+
+#| The title of a series
+has Str:D $.title is required;
+
+#| The description of a series
+has Str:D $.desc is required;
+
+#| The ids of the posts in the series, in series order
+has Int:D @.post-ids = [];