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_providers.py

@@ -120,13 +120,13 @@ class TestImageProvider:
             assert inputs["width"] == 1920
             assert inputs["height"] == 480
 
-    async def test_image_with_reference_image(
+    async def test_image_with_reference_image_flux1(
         self, project_dir: Path, image_bytes: bytes
     ) -> None:
         ref_path = project_dir / "ref.png"
         _ = ref_path.write_bytes(b"reference image data")
 
-        target_config = TargetConfig(prompt="Like this", reference_image="ref.png")
+        target_config = TargetConfig(prompt="Like this", reference_images=["ref.png"])
         bfl_result, mock_http = _make_bfl_mocks(image_bytes)
 
         with (
@@ -142,7 +142,7 @@ class TestImageProvider:
                 target_name="out.png",
                 target_config=target_config,
                 resolved_prompt="Like this",
-                resolved_model="flux-kontext-pro",
+                resolved_model="flux-pro-1.1",
                 project_dir=project_dir,
             )
 
@@ -151,6 +151,71 @@ class TestImageProvider:
             assert "image_prompt" in inputs
             assert inputs["image_prompt"] == encode_image_b64(ref_path)
 
+    async def test_image_with_reference_images_flux2(
+        self, project_dir: Path, image_bytes: bytes
+    ) -> None:
+        ref1 = project_dir / "ref1.png"
+        ref2 = project_dir / "ref2.png"
+        _ = ref1.write_bytes(b"ref1 data")
+        _ = ref2.write_bytes(b"ref2 data")
+
+        target_config = TargetConfig(
+            prompt="Combine", reference_images=["ref1.png", "ref2.png"]
+        )
+        bfl_result, mock_http = _make_bfl_mocks(image_bytes)
+
+        with (
+            patch("bulkgen.providers.image.BFLClient") as mock_cls,
+            patch("bulkgen.providers.image.httpx.AsyncClient") as mock_http_cls,
+        ):
+            mock_generate = AsyncMock(return_value=bfl_result)
+            mock_cls.return_value.generate = mock_generate
+            mock_http_cls.return_value = mock_http
+
+            provider = ImageProvider(api_key="test-key")
+            await provider.generate(
+                target_name="out.png",
+                target_config=target_config,
+                resolved_prompt="Combine",
+                resolved_model="flux-2-pro",
+                project_dir=project_dir,
+            )
+
+            call_args = mock_generate.call_args
+            inputs = call_args[0][1]
+            assert inputs["input_image"] == encode_image_b64(ref1)
+            assert inputs["input_image_2"] == encode_image_b64(ref2)
+
+    async def test_image_with_reference_image_kontext(
+        self, project_dir: Path, image_bytes: bytes
+    ) -> None:
+        ref_path = project_dir / "ref.png"
+        _ = ref_path.write_bytes(b"reference image data")
+
+        target_config = TargetConfig(prompt="Edit", reference_images=["ref.png"])
+        bfl_result, mock_http = _make_bfl_mocks(image_bytes)
+
+        with (
+            patch("bulkgen.providers.image.BFLClient") as mock_cls,
+            patch("bulkgen.providers.image.httpx.AsyncClient") as mock_http_cls,
+        ):
+            mock_generate = AsyncMock(return_value=bfl_result)
+            mock_cls.return_value.generate = mock_generate
+            mock_http_cls.return_value = mock_http
+
+            provider = ImageProvider(api_key="test-key")
+            await provider.generate(
+                target_name="out.png",
+                target_config=target_config,
+                resolved_prompt="Edit",
+                resolved_model="flux-kontext-pro",
+                project_dir=project_dir,
+            )
+
+            call_args = mock_generate.call_args
+            inputs = call_args[0][1]
+            assert inputs["input_image"] == encode_image_b64(ref_path)
+
     async def test_image_no_sample_url_raises(self, project_dir: Path) -> None:
         target_config = TargetConfig(prompt="x")