add favicon and site icon generation from uploaded images

Upload a source image (PNG, JPEG, or SVG) and get a complete favicon
setup: PNG variants at 32, 180, 192, 512px served from DB via
FaviconController with ETag caching, SVG favicon for vector sources,
dynamic site.webmanifest, and theme-color meta tag. Theme editor gains
a site icon section with "use logo as icon" toggle, dedicated icon
upload, short name, and background colour picker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod/media/favicon_variant.ex

@@ -0,0 +1,26 @@
+defmodule Berrypod.Media.FaviconVariant do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "favicon_variants" do
+    field :source_image_id, :binary_id
+    field :png_32, :binary
+    field :png_180, :binary
+    field :png_192, :binary
+    field :png_512, :binary
+    field :svg, :string
+    field :generated_at, :utc_datetime
+
+    timestamps(type: :utc_datetime)
+  end
+
+  @doc false
+  def changeset(variant, attrs) do
+    variant
+    |> cast(attrs, [:source_image_id, :png_32, :png_180, :png_192, :png_512, :svg, :generated_at])
+    |> validate_required([:source_image_id, :generated_at])
+  end
+end

lib/berrypod/media/image.ex

@@ -38,7 +38,7 @@ defmodule Berrypod.Media.Image do
       :variants_status
     ])
     |> validate_required([:image_type, :filename, :content_type, :file_size, :data])
-    |> validate_inclusion(:image_type, ~w(logo header product))
+    |> validate_inclusion(:image_type, ~w(logo header product icon))
     |> validate_number(:file_size, less_than: @max_file_size)
     |> detect_svg()
   end