Route direct image-edit requests through the img2img pipeline

Direct `models/infer` image requests were defaulting to `image.text2image`, which dropped reference inputs and produced unrelated generations. Route image-edit payloads to `image.img2img`, accept canonical `image` URLs during reference resolution, and preserve reference-aware prompt hints.

Constraint: The test server must continue accepting both canonical `image` and legacy alias fields from callers already in circulation
Rejected: Fix only reference URL parsing | direct overrides would still be misrouted as text2image before reference resolution runs
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Any future image-model route inference must inspect input shape before defaulting to text2image
Tested: `env GOCACHE=/tmp/popiartserver-gocache go test ./internal/server -run 'TestInferRouteKeyForModelRecognizesViduAsVideo|TestInferRouteKeyForModelRecognizesImageEditInputs|TestResolveImageToImageReferenceAcceptsCanonicalImageURL|TestGenerateEditedImageRefsUsesSeedreamImagesGenerationsEndpoint'`; deployed to `101.42.99.35` and verified Seedream gateway requests include `image`
Not-tested: Full manual validation against every non-Seedream image-edit provider

internal/server/artifact_upload_test.go

@@ -130,3 +130,42 @@ func TestArtifactUploadCreatesReadableArtifactForSourceArtifactID(t *testing.T)
 		t.Fatal("expected resolved reference content to match uploaded bytes")
 	}
 }
+
+func TestResolveImageToImageReferenceAcceptsCanonicalImageURL(t *testing.T) {
+	cfg := Config{
+		SQLitePath:    filepath.Join(t.TempDir(), "popiart.db"),
+		SkillhubDir:   makeEmptySkillhub(t),
+		SessionSecret: "test-secret",
+	}
+	server, err := NewWithConfig(cfg)
+	if err != nil {
+		t.Fatalf("NewWithConfig: %v", err)
+	}
+
+	imageBytes := tinyPNG(t)
+	refSrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "image/png")
+		_, _ = w.Write(imageBytes)
+	}))
+	defer refSrv.Close()
+
+	editRef, err := server.resolveImageToImageReference(context.Background(), &job{
+		UserID:      "user_test",
+		UpstreamKey: "sk-upstream",
+		SkillID:     "popiskill-image-img2img-basic-v1",
+	}, map[string]any{
+		"image": refSrv.URL + "/reference.png",
+	})
+	if err != nil {
+		t.Fatalf("resolveImageToImageReference: %v", err)
+	}
+	if editRef.URL != refSrv.URL+"/reference.png" {
+		t.Fatalf("expected resolved URL to match input image, got %q", editRef.URL)
+	}
+	if editRef.ContentType != "image/png" {
+		t.Fatalf("expected image/png, got %q", editRef.ContentType)
+	}
+	if !bytes.Equal(editRef.Content, imageBytes) {
+		t.Fatal("expected resolved canonical image content to match downloaded bytes")
+	}
+}