Take the signature, the expiry and the URL scheme off the image request (closes #314)

The image request now holds only its source and its transform, exactly what
the variant key covers. The /v1/image/ handler checks the allowlist and
verifies the signature itself with Signer.Verify(req, sig, expires). Both
image handlers keep the URL's expiry as a local value for Cache-Control. The
image service holds no signer, allowlist or AllowHTTP, and ValidateRequest is
gone. The fetcher takes the source and builds its URL with Source.URL from its
own AllowHTTP, so no URL string is built and parsed again. chooseAutoFormat
returns the format it chooses instead of writing into the request. The signed
URL format and every route's answers are unchanged.

Model: opus-5-5

internal/providers/providers.go

@@ -212,21 +212,15 @@ type imageServiceParams struct {
 }
 
 // newImageService builds the image service from the cache, the fetcher
-// and the processor, with the signing key, the allowlist and the upstream
-// settings of the config.
+// and the processor, with the largest upstream response the config allows.
 func newImageService(
 	_ fx.Lifecycle, params imageServiceParams,
 ) (*imgcache.Service, error) {
-	cfg := params.Config
-
 	return imgcache.NewService(&imgcache.ServiceConfig{
 		Cache:           params.Cache,
 		Fetcher:         params.Fetcher,
 		Processor:       params.Processor,
-		SigningKey:      cfg.SigningKey,
-		Allowlist:       cfg.AllowlistHosts,
-		AllowHTTP:       cfg.AllowHTTP,
-		MaxResponseSize: cfg.UpstreamMaxResponseSize,
+		MaxResponseSize: params.Config.UpstreamMaxResponseSize,
 		Logger:          params.Logger.Get(),
 	})
 }