add db unique constraint for tags

migrations/127_tag_name_unique.rb

@@ -0,0 +1,13 @@
+Sequel.migration do
+  up {
+    alter_table(:tags) do
+      add_unique_constraint :name
+    end   
+  }
+  
+  down {
+    alter_table(:tags) do
+      drop_constraint :tags_name_key
+    end   
+  }
+end