Refactor micropub specific things out to a slice

slices/micropub/commands/posts/add_syndication_source.rb

@@ -0,0 +1,18 @@
+module Micropub
+  module Commands
+    module Posts
+      class AddSyndicationSource
+        include Deps["repos.post_repo"]
+
+        def call(post_id, source, url)
+          post = post_repo.find!(post_id).to_h
+          syndication_sources = post[:syndication_sources] || {}
+          syndication_sources[source] = url
+          post[:syndication_sources] = syndication_sources
+
+          post_repo.update(post_id, post)
+        end
+      end
+    end
+  end
+end