Add palceholder post and update db init

lib/DB/PlaceholderPost.rakumod

@@ -0,0 +1,32 @@
+use v6.e.PREVIEW
+
+use Pandoc;
+use JSON::Class:auth<zef:vrurg>;
+
+use DB::Post;
+
+#| An empty placeholder post
+unit class PlaceholderPost does Post is json;
+
+#| Marker for disambiguation between post types in json representation, the
+#| cheaty way
+has Bool:D $.placeholder = True;
+
+method title(--> Str:D) {
+    "Placeholder Article"
+}
+
+#| Shortcut for creating an empty placeholder post
+method empty(--> PlaceholderPost:D) {
+    PlaceholderPost.new(
+        source => "/dev/null".IO,
+        posted-at => DateTime.now,
+        hidden => True,
+    )
+}
+
+# Override the all-slugs method to return an empty array, we don't want the
+# placeholder article to be refered to as anything but its id
+method all-slugs(--> Array[Str:D]) {
+    Array[Str:D].new
+}