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).

tests/test_config.py

@@ -45,7 +45,7 @@ class TestLoadConfig:
                     "width": 1920,
                     "height": 480,
                     "inputs": ["ref.png"],
-                    "reference_image": "ref.png",
+                    "reference_images": ["ref.png"],
                     "control_images": ["ctrl.png"],
                 },
                 "story.md": {
@@ -66,7 +66,7 @@ class TestLoadConfig:
         assert banner.model == "flux-dev"
         assert banner.width == 1920
         assert banner.height == 480
-        assert banner.reference_image == "ref.png"
+        assert banner.reference_images == ["ref.png"]
         assert banner.control_images == ["ctrl.png"]
 
         story = config.targets["story.md"]