Support loading model weights from local project folders

Add bgfilter/weights.py: resolve_model_source() maps a HuggingFace repo id
to a local folder under the weights dir (models/ by default, override with
BGFILTER_WEIGHTS_DIR) when one named after the repo basename exists; otherwise
the repo id is returned unchanged. Opt-in and backward compatible.

- vitmatte_infer.py / segmentation.py resolve model_name through it; anime-seg
  reads <dir>/isnetis.onnx directly instead of hf_hub_download when local.
- .gitignore: models/, model-cache/
- README: document bundling weights as plain project folders.

Verified: anime-seg loads from a local models/anime-seg/ folder offline.

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

bgfilter/weights.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+
+def weights_dir() -> Path:
+    """Directory scanned for locally-bundled model weights.
+
+    Defaults to ``models`` (relative to the working directory); override with the
+    ``BGFILTER_WEIGHTS_DIR`` environment variable.
+    """
+    return Path(os.environ.get("BGFILTER_WEIGHTS_DIR", "models"))
+
+
+def resolve_model_source(repo_id: str) -> str:
+    """Map a HuggingFace repo id to a local weights folder when one is present.
+
+    If ``<weights_dir>/<basename>`` exists it is returned (so models can be shipped
+    as plain folders inside the project, e.g. ``ZhengPeng7/BiRefNet`` ->
+    ``models/BiRefNet``); otherwise the repo id is returned unchanged so it loads
+    from the HF hub/cache as before. An absolute/existing path is passed through.
+    """
+    if os.path.isdir(repo_id):
+        return repo_id
+    local = weights_dir() / repo_id.split("/")[-1]
+    if local.is_dir():
+        return str(local)
+    return repo_id