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/auth.go

@@ -371,7 +371,7 @@ func (s *Handlers) buildGeneratedURL(r *http.Request, token, format string) stri
 
 	// Determine file extension for the trailing filename
 	ext := format
-	if ext == "" || ext == "orig" {
+	if ext == "" || ext == "orig" || ext == "auto" {
 		ext = "jpg" // Default extension
 	}