feat: enhance image optimization with on-demand JPEG fallbacks

Improve the image optimization pipeline with better compression and
smarter variant generation:

- Change to_lossless_webp → to_optimized_webp (lossy, quality 90)
- Auto-resize uploads larger than 2000px to save storage
- Skip pre-generating JPEG variants (~50% disk savings)
- Add on-demand JPEG generation for legacy browsers (<5% of users)
- Add /images/:id/variant/:width route for dynamic serving
- Add VariantCache to supervision tree for startup validation
- Add image_cache to static paths for disk-based serving

The pipeline now stores smaller WebP sources and generates AVIF/WebP
variants upfront, with JPEG generated only when legacy browsers request it.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme_web/controllers/image_controller.ex

@@ -25,29 +25,133 @@ defmodule SimpleshopThemeWeb.ImageController do
   end
 
   @doc """
-  Serves a thumbnail of an image if available, otherwise falls back to full image.
+  Serves a thumbnail of an image from the disk cache.
+
+  Thumbnails are generated by the background image optimization pipeline.
+  If the thumbnail doesn't exist on disk yet (still processing), generates
+  it on-demand and saves it for future requests.
   """
   def thumbnail(conn, %{"id" => id}) do
-    case Media.get_image(id) do
+    thumb_path = Media.get_thumbnail_path(id)
+
+    if File.exists?(thumb_path) do
+      # Serve from disk cache
+      conn
+      |> put_resp_content_type("image/jpeg")
+      |> put_resp_header("cache-control", "public, max-age=31536000, immutable")
+      |> put_resp_header("etag", ~s("#{id}-thumb"))
+      |> send_file(200, thumb_path)
+    else
+      # Thumbnail not yet generated - generate on-demand
+      case Media.get_image(id) do
+        nil ->
+          send_resp(conn, 404, "Image not found")
+
+        %{data: data} when is_binary(data) ->
+          case generate_thumbnail_on_demand(data, thumb_path) do
+            {:ok, binary} ->
+              conn
+              |> put_resp_content_type("image/jpeg")
+              |> put_resp_header("cache-control", "public, max-age=31536000, immutable")
+              |> put_resp_header("etag", ~s("#{id}-thumb"))
+              |> send_resp(200, binary)
+
+            {:error, _} ->
+              # Fallback to full image if thumbnail generation fails
+              conn
+              |> put_resp_content_type("image/webp")
+              |> put_resp_header("cache-control", "public, max-age=31536000, immutable")
+              |> send_resp(200, data)
+          end
+
+        %{data: nil} ->
+          send_resp(conn, 404, "Image data not available")
+      end
+    end
+  end
+
+  defp generate_thumbnail_on_demand(image_data, thumb_path) do
+    with {:ok, image} <- Image.from_binary(image_data),
+         {:ok, thumb} <- Image.thumbnail(image, 200),
+         {:ok, binary} <- Image.write(thumb, :memory, suffix: ".jpg", quality: 80) do
+      # Ensure cache directory exists and save for future requests
+      File.mkdir_p!(Path.dirname(thumb_path))
+      File.write!(thumb_path, binary)
+      {:ok, binary}
+    end
+  rescue
+    _ -> {:error, :thumbnail_generation_failed}
+  end
+
+  @supported_formats %{"avif" => :avif, "webp" => :webp, "jpg" => :jpg}
+
+  @doc """
+  Serves an image variant at the specified width and format.
+
+  Supports AVIF, WebP, and JPEG formats. If the variant doesn't exist on disk
+  (e.g., cache was deleted), it will be generated on-demand and cached.
+
+  JPEG variants are never pre-generated (to save disk space), so they are
+  always generated on first request.
+
+  ## URL format
+
+      /images/:id/variant/:width.:format
+
+  Examples:
+  - `/images/abc123/variant/800.avif`
+  - `/images/abc123/variant/400.webp`
+  - `/images/abc123/variant/1200.jpg`
+  """
+  def variant(conn, %{"id" => id, "width" => width_with_ext}) do
+    alias SimpleshopTheme.Images.Optimizer
+
+    with {width, format} <- parse_width_and_format(width_with_ext),
+         true <- width in Optimizer.all_widths(),
+         %{data: data} when is_binary(data) <- Media.get_image(id) do
+      case Optimizer.generate_variant_on_demand(data, id, width, format) do
+        {:ok, path} ->
+          conn
+          |> put_resp_content_type(format_content_type(format))
+          |> put_resp_header("cache-control", "public, max-age=31536000, immutable")
+          |> put_resp_header("etag", ~s("#{id}-#{width}.#{format}"))
+          |> send_file(200, path)
+
+        {:error, _reason} ->
+          send_resp(conn, 500, "Failed to generate variant")
+      end
+    else
+      :error ->
+        send_resp(conn, 400, "Invalid width or format")
+
+      false ->
+        send_resp(conn, 400, "Width not supported")
+
       nil ->
         send_resp(conn, 404, "Image not found")
 
-      %{thumbnail_data: thumbnail_data} = image when not is_nil(thumbnail_data) ->
-        conn
-        |> put_resp_content_type("image/jpeg")
-        |> put_resp_header("cache-control", "public, max-age=31536000, immutable")
-        |> put_resp_header("etag", ~s("#{image.id}-thumb"))
-        |> send_resp(200, thumbnail_data)
-
-      image ->
-        conn
-        |> put_resp_content_type(image.content_type)
-        |> put_resp_header("cache-control", "public, max-age=31536000, immutable")
-        |> put_resp_header("etag", ~s("#{image.id}"))
-        |> send_resp(200, image.data)
+      %{data: nil} ->
+        send_resp(conn, 404, "Image data not available")
     end
   end
 
+  defp parse_width_and_format(width_with_ext) do
+    case String.split(width_with_ext, ".") do
+      [width_str, ext] when is_map_key(@supported_formats, ext) ->
+        case Integer.parse(width_str) do
+          {width, ""} -> {width, @supported_formats[ext]}
+          _ -> :error
+        end
+
+      _ ->
+        :error
+    end
+  end
+
+  defp format_content_type(:avif), do: "image/avif"
+  defp format_content_type(:webp), do: "image/webp"
+  defp format_content_type(:jpg), do: "image/jpeg"
+
   @doc """
   Serves an SVG image recolored with the specified color.