feat: support multiple reference images with model-aware API mapping

Replace singular reference_image field with reference_images list to
support an arbitrary number of reference images. Map them to the correct
BFL API parameter names based on model family:
- flux-2-*: input_image, input_image_2, ..., input_image_8
- flux-kontext-*: input_image, input_image_2, ..., input_image_4
- flux 1.x: image_prompt (single)

BREAKING CHANGE: reference_image config field renamed to reference_images (list).

bulkgen/providers/image.py

@@ -18,6 +18,32 @@ def _encode_image_b64(path: Path) -> str:
     return base64.b64encode(path.read_bytes()).decode("ascii")
 
 
+# Parameter names for reference images, keyed by model prefix.
+_INPUT_IMAGE_KEYS = ["input_image"] + [f"input_image_{i}" for i in range(2, 9)]
+_IMAGE_PROMPT_KEYS = ["image_prompt"]
+
+
+def _ref_image_keys(model: str) -> list[str]:
+    """Return the ordered API parameter names for reference images."""
+    if model.startswith("flux-2-"):
+        return _INPUT_IMAGE_KEYS  # up to 8
+    if model.startswith("flux-kontext-"):
+        return _INPUT_IMAGE_KEYS[:4]  # up to 4
+    return _IMAGE_PROMPT_KEYS  # flux 1.x: single image_prompt
+
+
+def _add_reference_images(
+    inputs: dict[str, object],
+    reference_images: list[str],
+    model: str,
+    project_dir: Path,
+) -> None:
+    """Encode reference images and add them under the correct API keys."""
+    keys = _ref_image_keys(model)
+    for key, ref_name in zip(keys, reference_images, strict=False):
+        inputs[key] = _encode_image_b64(project_dir / ref_name)
+
+
 class ImageProvider(Provider):
     """Generates images via the BlackForestLabs API."""
 
@@ -44,9 +70,10 @@ class ImageProvider(Provider):
         if target_config.height is not None:
             inputs["height"] = target_config.height
 
-        if target_config.reference_image is not None:
-            ref_path = project_dir / target_config.reference_image
-            inputs["image_prompt"] = _encode_image_b64(ref_path)
+        if target_config.reference_images:
+            _add_reference_images(
+                inputs, target_config.reference_images, resolved_model, project_dir
+            )
 
         for control_name in target_config.control_images:
             ctrl_path = project_dir / control_name