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>

configs/default.yaml

@@ -3,6 +3,15 @@
 # like "#CFEFFF" sets it explicitly. Also overridable via --screen-color.
 screen_color: null
 
+chroma:
+  # true = flat-colour background (default). false = non-flat/complex background
+  # mode (--no-chroma): no colour key; segmentation alone drives the trimap (mode
+  # forced to "seg") and the colour-keyed stages are bypassed (auto-detect, hue
+  # split, chroma suppression, despill). The cross-check veto still runs if
+  # enabled, using its hue-free gate (cross_check.second_lo/hi). Needs
+  # segmentation.enabled + matting_method vitmatte.
+  enabled: true
+
 model:
   model_name: hustvl/vitmatte-base-composition-1k
   device: cpu
@@ -91,6 +100,11 @@ cross_check:
   feather_sigma: 2.0   # zone-boundary feather, px
   gate_lo: 0.70        # primary alpha below this -> fully exempt
   gate_hi: 0.95        # primary alpha above this -> fully vetoable
+  # Complex-background (no-chroma) mode only: no hue-defined suspect zone, so the
+  # veto requires the second opinion itself to be confidently near-empty -- full
+  # strength at/below second_lo, fading to none at/above second_hi.
+  second_lo: 0.15
+  second_hi: 0.40
 
 foreground:
   enabled: true