fix: propagate AllowHTTP to SourceURL() scheme selection

SourceURL() previously hardcoded https:// regardless of the AllowHTTP
config setting. This made testing with HTTP-only test servers impossible.

Add AllowHTTP field to ImageRequest and use it to determine the URL
scheme. The Service propagates the config setting to each request.

Fixes #1

internal/imgcache/imgcache.go

@@ -79,11 +79,18 @@ type ImageRequest struct {
 	Signature string
 	// Expires is the signature expiration timestamp
 	Expires time.Time
+	// AllowHTTP indicates whether HTTP (non-TLS) is allowed for this request
+	AllowHTTP bool
 }
 
-// SourceURL returns the full upstream URL to fetch
+// SourceURL returns the full upstream URL to fetch.
+// Uses http:// scheme when AllowHTTP is true, otherwise https://.
 func (r *ImageRequest) SourceURL() string {
-	url := "https://" + r.SourceHost + r.SourcePath
+	scheme := "https"
+	if r.AllowHTTP {
+		scheme = "http"
+	}
+	url := scheme + "://" + r.SourceHost + r.SourcePath
 	if r.SourceQuery != "" {
 		url += "?" + r.SourceQuery
 	}

internal/imgcache/service.go

@@ -21,6 +21,7 @@ type Service struct {
 	signer    *Signer
 	whitelist *HostWhitelist
 	log       *slog.Logger
+	allowHTTP bool
 }
 
 // ServiceConfig holds configuration for the image service.
@@ -68,6 +69,11 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 		log = slog.Default()
 	}
 
+	allowHTTP := false
+	if cfg.FetcherConfig != nil {
+		allowHTTP = cfg.FetcherConfig.AllowHTTP
+	}
+
 	return &Service{
 		cache:     cfg.Cache,
 		fetcher:   fetcher,
@@ -75,6 +81,7 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 		signer:    signer,
 		whitelist: NewHostWhitelist(cfg.Whitelist),
 		log:       log,
+		allowHTTP: allowHTTP,
 	}, nil
 }
 
@@ -83,6 +90,9 @@ var ErrNegativeCached = errors.New("request is in negative cache (recently faile
 
 // Get retrieves a processed image, fetching and processing if necessary.
 func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, error) {
+	// Propagate AllowHTTP setting to the request
+	req.AllowHTTP = s.allowHTTP
+
 	// Check negative cache first - skip fetching for recently-failed URLs
 	negHit, err := s.cache.checkNegativeCache(ctx, req)
 	if err != nil {

internal/imgcache/sourceurl_test.go

@@ -0,0 +1,44 @@
+package imgcache
+
+import "testing"
+
+func TestImageRequest_SourceURL_DefaultHTTPS(t *testing.T) {
+	req := &ImageRequest{
+		SourceHost:  "cdn.example.com",
+		SourcePath:  "/photos/cat.jpg",
+		SourceQuery: "v=2",
+	}
+
+	got := req.SourceURL()
+	want := "https://cdn.example.com/photos/cat.jpg?v=2"
+	if got != want {
+		t.Errorf("SourceURL() = %q, want %q", got, want)
+	}
+}
+
+func TestImageRequest_SourceURL_AllowHTTP(t *testing.T) {
+	req := &ImageRequest{
+		SourceHost: "localhost:8080",
+		SourcePath: "/photos/cat.jpg",
+		AllowHTTP:  true,
+	}
+
+	got := req.SourceURL()
+	want := "http://localhost:8080/photos/cat.jpg"
+	if got != want {
+		t.Errorf("SourceURL() = %q, want %q", got, want)
+	}
+}
+
+func TestImageRequest_SourceURL_AllowHTTPFalse(t *testing.T) {
+	req := &ImageRequest{
+		SourceHost: "cdn.example.com",
+		SourcePath: "/img.jpg",
+		AllowHTTP:  false,
+	}
+
+	got := req.SourceURL()
+	if got != "https://cdn.example.com/img.jpg" {
+		t.Errorf("SourceURL() = %q, want https scheme", got)
+	}
+}