feat: add Settings and Media contexts with theme settings schema

- Create settings table for site-wide key-value configuration
- Create images table for BLOB storage of logo/header images
- Add Setting schema with JSON/string/integer/boolean support
- Add ThemeSettings embedded schema with all theme options
- Add Settings context with get/put/update operations
- Add Media context for image uploads and retrieval
- Add Image schema with SVG detection and storage
- Add 9 curated theme presets (gallery, studio, boutique, etc.)
- Add comprehensive tests for Settings and Media contexts
- Add seeds with default Studio preset
- All tests passing (29 tests, 0 failures)

lib/simpleshop_theme/media/image.ex

@@ -0,0 +1,54 @@
+defmodule SimpleshopTheme.Media.Image do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "images" do
+    field :image_type, :string
+    field :filename, :string
+    field :content_type, :string
+    field :file_size, :integer
+    field :data, :binary
+    field :is_svg, :boolean, default: false
+    field :svg_content, :string
+    field :thumbnail_data, :binary
+
+    timestamps(type: :utc_datetime)
+  end
+
+  @max_file_size 5_000_000
+
+  @doc false
+  def changeset(image, attrs) do
+    image
+    |> cast(attrs, [:image_type, :filename, :content_type, :file_size, :data, :is_svg, :svg_content])
+    |> validate_required([:image_type, :filename, :content_type, :file_size, :data])
+    |> validate_inclusion(:image_type, ~w(logo header product))
+    |> validate_number(:file_size, less_than: @max_file_size)
+    |> detect_svg()
+  end
+
+  defp detect_svg(changeset) do
+    content_type = get_change(changeset, :content_type)
+
+    if content_type == "image/svg+xml" or String.ends_with?(get_change(changeset, :filename) || "", ".svg") do
+      changeset
+      |> put_change(:is_svg, true)
+      |> maybe_store_svg_content()
+    else
+      changeset
+    end
+  end
+
+  defp maybe_store_svg_content(changeset) do
+    case get_change(changeset, :data) do
+      nil ->
+        changeset
+
+      svg_binary when is_binary(svg_binary) ->
+        put_change(changeset, :svg_content, svg_binary)
+    end
+  end
+end