refactor site_updated, add initial site file tracking

migrations/050_add_site_files.rb

@@ -0,0 +1,17 @@
+Sequel.migration do
+  up {
+    DB.create_table! :site_files do
+      Integer  :site_id, index: true
+      String   :path
+      Bigint   :size
+      String   :sha1_hash
+      Boolean  :is_directory, default: false
+      DateTime :created_at
+      DateTime :updated_at
+    end
+  }
+
+  down {
+    DB.drop_table :site_files
+  }
+end