feat: add image optimizer module

- Add Optimizer module with lossless WebP conversion
- Generate responsive variants at [400, 800, 1200] widths
- Only create sizes <= source dimensions (no upscaling)
- Support AVIF, WebP, and JPEG output formats
- Add disk cache at priv/static/image_cache/
- Add comprehensive test suite (12 tests)
- Add image fixtures helper for testing

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

test/simpleshop_theme/images/optimizer_test.exs

@@ -0,0 +1,121 @@
+defmodule SimpleshopTheme.Images.OptimizerTest do
+  use SimpleshopTheme.DataCase, async: false
+
+  alias SimpleshopTheme.Images.Optimizer
+  import SimpleshopTheme.ImageFixtures
+
+  setup do
+    cleanup_cache()
+    on_exit(&cleanup_cache/0)
+    :ok
+  end
+
+  describe "applicable_widths/1" do
+    test "returns all widths for large source" do
+      assert [400, 800, 1200] = Optimizer.applicable_widths(1500)
+    end
+
+    test "excludes widths larger than source" do
+      assert [400, 800] = Optimizer.applicable_widths(900)
+      assert [400] = Optimizer.applicable_widths(500)
+    end
+
+    test "returns source width when smaller than minimum" do
+      assert [300] = Optimizer.applicable_widths(300)
+      assert [50] = Optimizer.applicable_widths(50)
+    end
+  end
+
+  describe "to_lossless_webp/1" do
+    test "converts image and returns dimensions" do
+      {:ok, webp, width, height} = Optimizer.to_lossless_webp(sample_jpeg())
+
+      assert is_binary(webp)
+      assert width == 1200
+      assert height == 800
+      # WebP magic bytes: RIFF....WEBP
+      assert <<"RIFF", _size::binary-size(4), "WEBP", _::binary>> = webp
+    end
+
+    test "returns error for invalid data" do
+      assert {:error, _} = Optimizer.to_lossless_webp("not an image")
+    end
+  end
+
+  describe "process_for_image/1" do
+    test "generates all variants for 1200px image" do
+      image = image_fixture(%{source_width: 1200, source_height: 800})
+
+      assert {:ok, [400, 800, 1200]} = Optimizer.process_for_image(image.id)
+
+      for w <- [400, 800, 1200], fmt <- [:avif, :webp, :jpg] do
+        assert File.exists?(cache_path(image.id, w, fmt)),
+               "Missing #{w}.#{fmt}"
+      end
+
+      assert File.exists?(cache_path(image.id, "thumb", :jpg))
+    end
+
+    test "generates only applicable widths for smaller image" do
+      # Create fixture with smaller source width
+      {:ok, webp, _w, _h} = Optimizer.to_lossless_webp(sample_jpeg())
+
+      image =
+        %SimpleshopTheme.Media.Image{}
+        |> SimpleshopTheme.Media.Image.changeset(%{
+          image_type: "product",
+          filename: "small.jpg",
+          content_type: "image/webp",
+          file_size: byte_size(webp),
+          data: webp,
+          source_width: 600,
+          source_height: 400,
+          variants_status: "pending",
+          is_svg: false
+        })
+        |> Repo.insert!()
+
+      assert {:ok, [400]} = Optimizer.process_for_image(image.id)
+
+      assert File.exists?(cache_path(image.id, 400, :avif))
+      refute File.exists?(cache_path(image.id, 800, :avif))
+    end
+
+    test "skips SVG images" do
+      image = svg_fixture()
+      assert {:ok, :svg_skipped} = Optimizer.process_for_image(image.id)
+    end
+
+    test "returns error for missing image" do
+      assert {:error, :not_found} = Optimizer.process_for_image(Ecto.UUID.generate())
+    end
+
+    test "is idempotent - skips existing files" do
+      image = image_fixture(%{source_width: 1200, source_height: 800})
+      {:ok, _} = Optimizer.process_for_image(image.id)
+
+      path = cache_path(image.id, 400, :avif)
+      {:ok, %{mtime: mtime1}} = File.stat(path)
+
+      Process.sleep(1100)
+      {:ok, _} = Optimizer.process_for_image(image.id)
+
+      {:ok, %{mtime: mtime2}} = File.stat(path)
+      assert mtime1 == mtime2, "File was regenerated"
+    end
+  end
+
+  describe "disk_variants_exist?/2" do
+    test "returns true when all variants exist" do
+      image = image_fixture(%{source_width: 1200, source_height: 800})
+      {:ok, _} = Optimizer.process_for_image(image.id)
+
+      assert Optimizer.disk_variants_exist?(image.id, 1200)
+    end
+
+    test "returns false when variants missing" do
+      image = image_fixture(%{source_width: 1200, source_height: 800})
+      refute Optimizer.disk_variants_exist?(image.id, 1200)
+    end
+  end
+end