test: show quality and fit are not covered by the URL signature

Add Quality and FitMode fields to signature.Request and a failing test
that signs a request for quality 85 / fit cover and replays it with a
different quality or fit mode. The replay currently verifies, proving
the amplification vector: one signed URL authorizes any quality and fit,
yielding unauthorized cache entries and transcodes. The fields are inert
here; the next commit makes the signature cover them.

model: claude-opus-4-8

internal/signature/quality_fit_test.go

@@ -0,0 +1,68 @@
+package signature_test
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"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) *signature.Request {
+	req := &signature.Request{
+		SourceHost: testHost,
+		SourcePath: testPath,
+		Width:      800,
+		Height:     600,
+		Format:     testFormatWebP,
+		Quality:    85,
+		FitMode:    "cover",
+		Expires:    time.Now().Add(1 * time.Hour),
+	}
+	req.Signature = signer.Sign(req)
+
+	return req
+}
+
+// TestSigner_Verify_QualityAndFitAreSigned proves that quality and fit are
+// covered by the signature: a URL signed for one quality or fit mode must
+// not verify when replayed with a different quality or fit mode. This is the
+// amplification vector from the issue — one signed URL replayed across many
+// quality and fit values yields many unauthorized cache entries and
+// transcodes — so it must be rejected.
+func TestSigner_Verify_QualityAndFitAreSigned(t *testing.T) {
+	t.Parallel()
+
+	signer := signature.New("test-secret-key")
+
+	cases := []struct {
+		name   string
+		tamper func(r *signature.Request)
+	}{
+		{
+			name:   "replayed with different quality",
+			tamper: func(r *signature.Request) { r.Quality = 40 },
+		},
+		{
+			name:   "replayed with different fit mode",
+			tamper: func(r *signature.Request) { r.FitMode = "contain" },
+		},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := signedQualityFitRequest(signer)
+			tt.tamper(req)
+
+			err := signer.Verify(req)
+			if !errors.Is(err, signature.ErrInvalid) {
+				t.Errorf("Verify() = %v, want %v", err, signature.ErrInvalid)
+			}
+		})
+	}
+}