First pass at uniformed types, specs and docs

- spec public functions (skipping controllers, views, live views and
components)
- use types instead of explanations in docs
- remove redundant docs
- fix typos

lib/music_library/assets.ex

@@ -8,6 +8,7 @@ defmodule MusicLibrary.Assets do
   Store any file type - the responsibility to correctly populate format and
   properties is left to the caller.
   """
+  @spec store(map()) :: {:ok, Asset.t()} | {:error, Ecto.Changeset.t()}
   def store(params) do
     %Asset{}
     |> Asset.changeset(params)
@@ -17,20 +18,24 @@ defmodule MusicLibrary.Assets do
   @doc """
   Store image files - properties will be computed automatically.
   """
+  @spec store_image(map()) :: {:ok, Asset.t()} | {:error, Ecto.Changeset.t()}
   def store_image(params) do
     %Asset{}
     |> Asset.image_changeset(params)
     |> Repo.insert(on_conflict: :nothing, returning: true)
   end
 
+  @spec get(String.t()) :: Asset.t() | nil
   def get(hash) do
     Repo.get(Asset, hash)
   end
 
+  @spec get!(String.t()) :: Asset.t()
   def get!(hash) do
     Repo.get!(Asset, hash)
   end
 
+  @spec total_content_size() :: non_neg_integer() | nil
   def total_content_size do
     q =
       from p in Asset, select: sum(fragment("length(content)"))
@@ -38,6 +43,7 @@ defmodule MusicLibrary.Assets do
     Repo.one(q)
   end
 
+  @spec track_total_content_size() :: :ok
   def track_total_content_size do
     :telemetry.execute(
       [:music_library, :assets],
@@ -46,6 +52,7 @@ defmodule MusicLibrary.Assets do
     )
   end
 
+  @spec track_total_cache_size() :: :ok
   def track_total_cache_size do
     :telemetry.execute(
       [:music_library, :assets],