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>

README.md

@@ -217,13 +217,13 @@ Config comes from `BGFILTER_CONFIG` (defaults to `configs/default.yaml` when pre
 caps input size (default ~4MP → `413`); `BGFILTER_PRELOAD=1` loads the default models at
 startup. Run a single worker (`--workers 1`) — each worker loads its own copy of the models.
 
-Unlike the CLI, the service defaults the **cross-model veto to off** — it adds one
-extra large-model inference (~35 s/image on CPU, 14 s → 49 s total). Enable it
-per request with the `cross_check` form field, or flip the service default with
-`BGFILTER_CROSS_CHECK=1` (worth it on GPU or when hair-gap residue matters more
-than latency); `/healthz` reports the default as `crossCheck`. The cross-check
-model is lazy-loaded: it is never preloaded at startup, so the first request that
-enables it also pays the one-time model load (~1 GB) on top of the inference.
+Like the CLI, the service runs the **cross-model veto by default** (config
+default: on). With reuse-as-seg it replaces the primary seg forward rather than
+adding to it, so it is not the latency hit it once was; its HR-matting model is
+preloaded with the rest of the warm set (and, on the birefnet backend, the now-
+redundant primary segmenter is skipped at startup). Set `BGFILTER_CROSS_CHECK=0`
+to turn it off service-wide, or use the per-request `cross_check` form field to
+override either way; `/healthz` reports the default as `crossCheck`.
 
 ## Quality Check