feat: ensure image output format matches file extension

- Add hokusai/image.py with Pillow-based format detection and conversion
- Pass output_format to OpenAI gpt-image models and BFL API
- Convert mismatched images after provider writes (fallback for all providers)
- Handle RGBA-to-RGB flattening for JPEG targets
- Add Pillow dependency

hokusai/builder.py

@@ -19,6 +19,7 @@ from hokusai.config import (
     ProjectConfig,
 )
 from hokusai.graph import build_graph, get_build_order, get_subgraph_for_target
+from hokusai.image import ensure_image_format
 from hokusai.prompt import extract_placeholder_files, resolve_prompt
 from hokusai.providers import Provider
 from hokusai.providers.blackforest import BlackForestProvider
@@ -170,6 +171,7 @@ async def _build_single_target(
 
     if isinstance(target_cfg, DownloadTargetConfig):
         await _download_target(target_name, target_cfg, project_dir)
+        ensure_image_format(output_path)
         return
 
     model_info = resolve_model(target_name, target_cfg, config.defaults)
@@ -184,6 +186,9 @@ async def _build_single_target(
         project_dir=project_dir,
     )
 
+    # Ensure the output image format matches the file extension.
+    ensure_image_format(output_path)
+
 
 def _cleanup_stale_targets(
     config: ProjectConfig,