Switch to govips for native CGO image processing

- Replace gen2brain/avif, gen2brain/webp, disintegration/imaging with govips
- govips uses libvips via CGO for fast native image processing
- Add libheif-dev to Dockerfile for AVIF support
- Add docker-test Makefile target for running tests in Docker
- Update processor.go to use vips API for decode, resize, encode
- Add TestMain to initialize/shutdown vips in tests
- Remove WASM-based libraries (gen2brain) in favor of native codecs

Performance improvement: AVIF encoding now uses native libheif instead of
WASM, significantly reducing encoding time for large images.

internal/imgcache/processor_test.go

@@ -10,8 +10,17 @@ import (
 	"io"
 	"os"
 	"testing"
+
+	"github.com/davidbyttow/govips/v2/vips"
 )
 
+func TestMain(m *testing.M) {
+	vips.Startup(nil)
+	code := m.Run()
+	vips.Shutdown()
+	os.Exit(code)
+}
+
 // createTestJPEG creates a simple test JPEG image.
 func createTestJPEG(t *testing.T, width, height int) []byte {
 	t.Helper()