Re-enable Credo ModuleDoc check

Closes #108

lib/music_library/assets/cache.ex

@@ -1,4 +1,8 @@
 defmodule MusicLibrary.Assets.Cache do
+  @moduledoc """
+  ETS-based asset cache with TTL for serving frequently accessed images.
+  """
+
   @spec new() :: :ets.table()
   def new do
     :ets.new(__MODULE__, [:named_table, :public, :compressed, read_concurrency: true])

lib/music_library/assets/image.ex

@@ -1,4 +1,8 @@
 defmodule MusicLibrary.Assets.Image do
+  @moduledoc """
+  Image processing via Vix (libvips) for covers and artist images.
+  """
+
   alias Vix.Vips.{Image, Operation}
 
   fallback_path = Application.app_dir(:music_library, ["priv", "image-not-found.jpg"])

lib/music_library/assets/transform.ex

@@ -1,4 +1,8 @@
 defmodule MusicLibrary.Assets.Transform do
+  @moduledoc """
+  Represents an image transformation (hash + target width) for asset serving.
+  """
+
   @derive JSON.Encoder
   defstruct [:hash, :width]