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/handlers/image_errors_internal_test.go

@@ -295,10 +295,9 @@ func TestHandleImage_AllowlistOrSignature(t *testing.T) {
 // host can be, and checks that each is refused with 400 naming the host as
 // decoded: allowlistedHost with a character after it that a URL host cannot
 // hold, percent-encoded, an IPv6 address without brackets, and brackets
-// around something that is not an IPv6 address. A "?" or "#" after
-// allowlistedHost would otherwise end the host when the source URL is read,
-// so the request would pass the allowlist and be fetched and cached under a
-// host that is not the one fetched.
+// around something that is not an IPv6 address. Later steps put the host
+// unparsed into the allowlist check, the cache key and the upstream URL, so
+// such a host is refused first.
 func TestHandleImage_InvalidHost_Returns400(t *testing.T) {
 	t.Parallel()
 
@@ -369,10 +368,9 @@ func TestHandleImage_HostForms_Served(t *testing.T) {
 
 // TestHandleImage_HashInSourcePathOrQuery_Returns400 sends image URLs whose
 // source path or source query holds a "#", written as %23, and checks that
-// each is refused with 400, the one signed for that source path included. The
-// source URL would otherwise end at the "#" when it is fetched, so each text
-// after it would fetch the image at photoPath and cache it under keys of its
-// own.
+// each is refused with 400, the one signed for that source path included.
+// A "#" starts a URL's fragment, so neither the source path nor the source
+// query may hold one.
 func TestHandleImage_HashInSourcePathOrQuery_Returns400(t *testing.T) {
 	t.Parallel()