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/segmentation.py

@@ -1,9 +1,12 @@
 from __future__ import annotations
 
+import os
+
 import numpy as np
 from PIL import Image
 
 from .settings import SegmentationSettings
+from .weights import resolve_model_source
 
 
 class BiRefNetSegmenter:
@@ -35,7 +38,7 @@ class BiRefNetSegmenter:
 
         self.torch = torch
         self.model = AutoModelForImageSegmentation.from_pretrained(
-            settings.model_name, trust_remote_code=True
+            resolve_model_source(settings.model_name), trust_remote_code=True
         )
         self.model.eval()
         self.model.float()  # checkpoint ships as fp16; force fp32 to match inputs
@@ -91,7 +94,11 @@ class AnimeSegSegmenter:
                 "pip install onnxruntime huggingface_hub"
             ) from exc
 
-        model_file = hf_hub_download(settings.model_name, "isnetis.onnx")
+        source = resolve_model_source(settings.model_name)
+        if os.path.isdir(source):
+            model_file = os.path.join(source, "isnetis.onnx")
+        else:
+            model_file = hf_hub_download(source, "isnetis.onnx")
         providers = ["CPUExecutionProvider"]
         if settings.device == "cuda" and "CUDAExecutionProvider" in ort.get_available_providers():
             providers = ["CUDAExecutionProvider", "CPUExecutionProvider"]