Serve the format auto, chosen from the Accept header (closes #88)

auto is a format in the /v1/image/ path, an encrypted URL's token and
the generator page. Once the signature or token is checked, pixa
chooses AVIF when Accept names image/avif, else WebP when it names
image/webp, else JPEG when the most specific of image/jpeg, image/* and
*/* allows it or Accept is absent. q=0 refuses a format; a header
allowing none of the three answers 406, one that does not parse 400.
The signature and token cover auto itself; the cache key and ETag use
the chosen format. Answers from then on carry Vary: Accept.

Model: opus-5-5

internal/handlers/imageenc.go

@@ -30,7 +30,7 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		start := time.Now()
 
 		req, ok := s.parseImageEncRequest(w, r)
-		if !ok {
+		if !ok || !s.chooseAutoFormat(w, r, req) {
 			return
 		}