Postgres-backed implementation of ExDav.Storage. Owns its own users, collections, resources, and tombstones tables; the Repo module is configured via config :ex_dav, repo: MyApp.Repo.

Returns plain maps to keep the DAV plugs agnostic of Ecto schemas. Each collection is shaped as %{name, displayname, description, kind, properties, ctag, resources} and each resource as %{name, body, content_type, etag, uid}.