feat(webp): add WebP streaming and snapshot APIs

- implement WebP streaming with multipart support
- add snapshot endpoint for WebP format with quality options
- introduce WebP encoding using pure Go library without FFmpeg
- update documentation and links for new WebP features

pkg/webp/writer.go

@@ -0,0 +1,38 @@
+package webp
+
+import (
+	"io"
+	"net/http"
+	"strconv"
+)
+
+const header = "--frame\r\nContent-Type: image/webp\r\nContent-Length: "
+
+// Writer writes multipart WebP frames to an HTTP response.
+type Writer struct {
+	wr  io.Writer
+	buf []byte
+}
+
+// NewWriter creates a Writer that sets the multipart Content-Type header.
+func NewWriter(w io.Writer) *Writer {
+	h := w.(http.ResponseWriter).Header()
+	h.Set("Content-Type", "multipart/x-mixed-replace; boundary=frame")
+	return &Writer{wr: w, buf: []byte(header)}
+}
+
+func (w *Writer) Write(p []byte) (n int, err error) {
+	w.buf = w.buf[:len(header)]
+	w.buf = append(w.buf, strconv.Itoa(len(p))...)
+	w.buf = append(w.buf, "\r\n\r\n"...)
+	w.buf = append(w.buf, p...)
+	w.buf = append(w.buf, "\r\n"...)
+
+	if _, err = w.wr.Write(w.buf); err != nil {
+		return 0, err
+	}
+
+	w.wr.(http.Flusher).Flush()
+
+	return len(p), nil
+}