Align default ViTMatte model to base; make --config optional

The ModelSettings.model_name dataclass default was still vitmatte-small (the
phase-1 "start small" choice) while configs/default.yaml used vitmatte-base, so a
bare run (no --config) silently differed from every documented example. Set the
dataclass default to base too: the built-in defaults now match default.yaml
field-for-field, so --config is only needed after editing that file to tune
parameters. README examples drop the now-redundant --config.

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

bgfilter/settings.py

@@ -90,7 +90,7 @@ class DespillSettings:
 
 @dataclass(frozen=True)
 class ModelSettings:
-    model_name: str = "hustvl/vitmatte-small-composition-1k"
+    model_name: str = "hustvl/vitmatte-base-composition-1k"
     device: str = "cpu"
     matting_method: str = "vitmatte"
     fallback_to_chroma_alpha: bool = False