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/despill.py

@@ -10,7 +10,7 @@ from .settings import DespillSettings
 def despill(
     rgb: np.ndarray,
     alpha: np.ndarray,
-    model: BackgroundModel,
+    model: BackgroundModel | None,
     settings: DespillSettings,
 ) -> tuple[np.ndarray, np.ndarray]:
     """Remove background-colour spill from the foreground.
@@ -24,7 +24,8 @@ def despill(
 
     Returns the corrected RGB (uint8) and the per-pixel despill weight (float32).
     """
-    if not settings.enabled:
+    # model is None with chroma disabled: no key colour means no spill direction.
+    if not settings.enabled or model is None:
         return rgb.copy(), np.zeros(alpha.shape, dtype=np.float32)
 
     bg_lab = np.asarray(model.lab_center, dtype=np.float32)