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/signature/quality_fit_test.go

@@ -9,10 +9,12 @@ import (
 	"sneak.berlin/go/pixa/internal/signature"
 )
 
-// signedQualityFitRequest returns a request signed for quality 85 and fit
-// mode "cover", the effective defaults the handler applies before
-// verification.
-func signedQualityFitRequest(signer *signature.Signer) *imagerequest.Request {
+// signedQualityFitRequest returns a request, its signature for quality 85 and
+// fit mode "cover", the effective defaults the handler applies before
+// verification, and the expiry it was signed with.
+func signedQualityFitRequest(
+	signer *signature.Signer,
+) (*imagerequest.Request, string, time.Time) {
 	req := &imagerequest.Request{
 		Source: imagerequest.Source{
 			Host: testHost,
@@ -24,11 +26,10 @@ func signedQualityFitRequest(signer *signature.Signer) *imagerequest.Request {
 			Quality: 85,
 			FitMode: testFitCover,
 		},
-		Expires: time.Now().Add(1 * time.Hour),
 	}
-	req.Signature = signer.Sign(req)
+	expires := time.Now().Add(1 * time.Hour)
 
-	return req
+	return req, signer.Sign(req, expires), expires
 }
 
 // TestSigner_Verify_QualityAndFitAreSigned proves that quality and fit are
@@ -60,10 +61,10 @@ func TestSigner_Verify_QualityAndFitAreSigned(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			req := signedQualityFitRequest(signer)
+			req, sig, expires := signedQualityFitRequest(signer)
 			tt.tamper(req)
 
-			err := signer.Verify(req)
+			err := signer.Verify(req, sig, expires)
 			if !errors.Is(err, signature.ErrInvalid) {
 				t.Errorf("Verify() = %v, want %v", err, signature.ErrInvalid)
 			}