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

internal/webp/webp_test.go

@@ -0,0 +1,16 @@
+package webp
+
+import (
+	"testing"
+)
+
+func TestInit(t *testing.T) {
+	// Verify Init() runs without panicking and registers API endpoints.
+	// api.HandleFunc registrations are idempotent so calling Init multiple times is safe.
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("Init() panicked: %v", r)
+		}
+	}()
+	Init()
+}