Add complex-background matting mode to the service via background_mode

Backports master's non-flat matting (chroma.enabled: false + the hue-free
cross-check gate) into server-edition, and exposes it over HTTP without
surfacing the internal "chroma" wording: /remove-background gains a
background_mode form field (flat, default | complex). complex maps to
chroma disabled -- no colour key, segmentation alone drives the trimap and
every colour-keyed stage (auto-detect, hue split, chroma suppression,
despill) is bypassed. The cross-check veto still works in complex mode via
its second-opinion-confidence gate (cross_check.second_lo/hi) but defaults
OFF there (it costs the HR-matting forward); an explicit cross_check=on
re-enables it.

No new model weights: complex mode reuses the already-provisioned BiRefNet
seg + ViTMatte (+ optional HR-matting cross-check). Flat mode is unchanged
(bit-identical), and server-edition's own extras (cross_check.lock,
foreground.use_gpu CuPy path) are preserved -- the port is surgical, not a
copy of master's files.

- settings: ChromaSettings.enabled, CrossCheckSettings.second_lo/hi
- config: override_settings chroma passthrough
- despill/foreground: model=None safe guards (foreground keeps GPU path)
- alpha_post: cross_check_alpha hue-free gate when proj is None
- pipeline: _process_rgb complex branch (seg-only trimap, skip colour stages)
- service/app: process(chroma=), background_mode field, complex-defaults-off
  cross-check, X-BGFilter-Background-Mode header
- cli: --chroma/--no-chroma
- configs/docs: gpu.yaml + default.yaml + README/README_ZH

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

bgfilter/config.py

@@ -99,6 +99,9 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     }
     if overrides.get("trimap_mode") is not None:
         trimap_updates["mode"] = overrides["trimap_mode"]
+    chroma_updates: dict[str, Any] = {}
+    if overrides.get("chroma") is not None:
+        chroma_updates["enabled"] = overrides["chroma"]
     despill_updates: dict[str, Any] = {}
     if overrides.get("despill") is not None:
         despill_updates["enabled"] = overrides["despill"]
@@ -130,7 +133,7 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
         else settings.screen_color
     )
     return PipelineSettings(
-        chroma=chroma,
+        chroma=_update_dataclass(chroma, chroma_updates),
         trimap=_update_dataclass(trimap, trimap_updates),
         alpha_post=alpha_post,
         cross_check=_update_dataclass(settings.cross_check, cross_check_updates),