Default to BiRefNet + CPU; add --seg-backend to pick the segmenter

- Segmentation backend now defaults to birefnet (ZhengPeng7/BiRefNet); anime-seg is
  selected at runtime with --seg-backend anime-seg, which also swaps in the matching
  weights (unknown backends raise). Config files stay for detailed tuning, not
  backend selection.
- Default device is now cpu everywhere (dataclass defaults, default.yaml,
  birefnet.yaml); pass --device cuda for GPU. Avoids a hard CUDA-required failure on
  machines without a CUDA-enabled PyTorch build.
- Docs: README + workflow status note updated (backend via --seg-backend, CPU
  default, examples no longer force --device cuda).

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

bgfilter/cli.py

@@ -30,6 +30,7 @@ def main(
     fg_safe_radius_ratio: float | None = typer.Option(None, "--fg-safe-radius-ratio", min=0.0),
     despill: bool | None = typer.Option(None, "--despill/--no-despill"),
     trimap_mode: str | None = typer.Option(None, "--trimap-mode", help="Trimap mode (segmentation pipeline): directional | seg | directional-hard-bg"),
+    seg_backend: str | None = typer.Option(None, "--seg-backend", help="Segmentation backend: birefnet (default) | anime-seg"),
 ) -> None:
     try:
         settings = override_settings(
@@ -45,6 +46,7 @@ def main(
             fg_safe_radius_ratio=fg_safe_radius_ratio,
             despill=despill,
             trimap_mode=trimap_mode,
+            seg_backend=seg_backend,
         )
 
         if input_dir is not None: