Add anime-seg segmentation backend (SkyTNT ISNet)

Add AnimeSegSegmenter (skytnt/anime-seg ISNet ONNX via onnxruntime, no remote
code) and a make_segmenter factory selected by SegmentationSettings.backend
("birefnet" | "anime-seg"). The ONNX output is already 0..1, so it slots into the
same soft-mask interface BiRefNetSegmenter uses.

On the anime samples anime-seg recovers more and more-coherent hair wisps than
BiRefNet (TestImage2 shoulder rescue: added px 1886 -> 3278, largest connected
component 147 -> 454), as expected from an anime-trained model. Default backend
stays birefnet.

Adds onnxruntime to requirements.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bgfilter/settings.py

@@ -104,6 +104,7 @@ class ModelSettings:
 @dataclass(frozen=True)
 class SegmentationSettings:
     enabled: bool = True
+    backend: str = "birefnet"  # "birefnet" or "anime-seg"
     model_name: str = "ZhengPeng7/BiRefNet"
     device: str = "cuda"
     input_size: int = 1024