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/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)
+	}
+}