Nice proposal! Medium is a pretty app and should demo well.
Schema
Naming: responses.stories_id should be story_id, taggings.post_id should be taggings.story_id
Indexes: Add these to your foreign keys. Make note of any uniqueness constraints.
Views
Nav view
Missing wireframe for this view
You'll probably also want a SearchBar view (and probably a SearchResults view as well)
Home view
How are stories displayed here? Are you going to use the full StoryShow view, or a StoriesIndex with condensed StoriesIndexItem subviews?
What's going on with the tags on the right? Will that filter the stories in the feed? Add tags to our new story? Are you going to have views for that? (TagsIndex and TagsIndexItem would be good candidates)
Timeline
Your bookmark button and "bookmarked stories" page belong in the same phase, not two different ones. Same goes for following.
Nice proposal! Medium is a pretty app and should demo well.
Schema
responses.stories_idshould bestory_id,taggings.post_idshould betaggings.story_idViews
Timeline