Implement AVIF encoding support

- Add AVIF case to encode() using gen2brain/avif
- Add AVIF to SupportedOutputFormats()
- Use default encoder speed of 6 for balance of speed/quality

internal/imgcache/processor.go

@@ -133,6 +133,7 @@ func (p *ImageProcessor) SupportedOutputFormats() []ImageFormat {
 		FormatPNG,
 		FormatGIF,
 		FormatWebP,
+		FormatAVIF,
 	}
 }
 
@@ -218,6 +219,9 @@ func (p *ImageProcessor) resize(img image.Image, width, height int, fit FitMode)
 
 const defaultQuality = 85
 
+// avifEncoderSpeed is the default AVIF encoder speed (0-10, higher = faster, lower quality).
+const avifEncoderSpeed = 6
+
 // encode encodes an image to the specified format.
 func (p *ImageProcessor) encode(img image.Image, format ImageFormat, quality int) ([]byte, error) {
 	if quality <= 0 {
@@ -256,7 +260,14 @@ func (p *ImageProcessor) encode(img image.Image, format ImageFormat, quality int
 		}
 
 	case FormatAVIF:
-		return nil, fmt.Errorf("%w: AVIF encoding not supported", ErrUnsupportedOutputFormat)
+		options := goavif.Options{
+			Quality: quality,
+			Speed:   avifEncoderSpeed,
+		}
+		err := goavif.Encode(&buf, img, options)
+		if err != nil {
+			return nil, err
+		}
 
 	default:
 		return nil, fmt.Errorf("unsupported output format: %s", format)