Make cross-check and reuse-as-seg the service default

Cross-check + reuse-as-seg are already the config/CLI default; align the HTTP
service with them instead of forcing cross-check off. The earlier off-by-default
was to avoid the extra inference, but reuse-as-seg now makes that forward double
as the seg mask (birefnet backend) -- it replaces the primary seg model rather
than adding to it, so the cost concern is gone.

- app.py: honour the config default (on); BGFILTER_CROSS_CHECK still forces
  either way, unset = config.
- service.py preload: warm the cross-check HR-matting model, and skip the now-
  redundant primary segmenter when reuse covers segmentation.
- README / DEPLOY(_ZH): document cross-check as default-on, BiRefNet_HR-matting
  as required (not optional), BGFILTER_CROSS_CHECK=0 to disable.

Verified: service defaults cross_check on, env forces both ways, preload picks
cross-checker and skips the primary segmenter.

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

app.py

@@ -62,12 +62,14 @@ def _build_settings():
     if device:
         # Covers both settings.model.device and settings.segmentation.device.
         settings = override_settings(settings, device=device)
-    # The cross-model veto adds one extra large-model inference (~35 s/image on
-    # CPU), so the service defaults it OFF regardless of the config default;
-    # opt back in with BGFILTER_CROSS_CHECK=1. Other cross_check params (model,
-    # thresholds) still come from the config.
-    cross_check = os.environ.get("BGFILTER_CROSS_CHECK", "0").strip().lower() in ("1", "true", "yes", "on")
-    settings = override_settings(settings, cross_check=cross_check)
+    # The cross-model veto is default behaviour (config default: on). With
+    # reuse-as-seg it replaces the primary seg forward rather than adding to it,
+    # so the earlier CPU-cost concern no longer applies. BGFILTER_CROSS_CHECK
+    # can still force it either way; unset = honour the config.
+    cc_env = os.environ.get("BGFILTER_CROSS_CHECK")
+    if cc_env is not None and cc_env.strip() != "":
+        cross_check = cc_env.strip().lower() in ("1", "true", "yes", "on")
+        settings = override_settings(settings, cross_check=cross_check)
     return settings