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/config.py

@@ -34,7 +34,7 @@ class TargetConfig(BaseModel):
     prompt: str
     model: str | None = None
     inputs: list[str] = []
-    reference_image: str | None = None
+    reference_images: list[str] = []
     control_images: list[str] = []
     width: int | None = None
     height: int | None = None