Add per-request cross_check toggle to the HTTP service (lazy-loaded)

POST /remove-background gains an optional cross_check form field
(1/0/true/false/yes/no/on/off; empty = service default). The effective
state is reported in the X-BGFilter-Cross-Check response header.

The cross-check model is now never preloaded at startup: the first
request that enables it pays the one-time load (measured ~8 s on top of
the ~50 s cc inference), after which the instance is cached and shared.

Verified with a functional test against PipelineManager: cc-off requests
never load the model, cc=1 lazy-loads then reuses the cached instance,
and omitting the field falls back to the service default.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app.py

@@ -129,6 +129,20 @@ def _normalize_seg_model(value: str | None) -> str | None:
     return v
 
 
+def _normalize_cross_check(value: str | None) -> bool | None:
+    """None/empty -> use the service default; otherwise a boolean string."""
+    if value is None:
+        return None
+    v = value.strip().lower()
+    if v == "":
+        return None
+    if v in ("1", "true", "yes", "on"):
+        return True
+    if v in ("0", "false", "no", "off"):
+        return False
+    raise HTTPException(status_code=400, detail="invalid cross_check")
+
+
 def _decode_image(data: bytes) -> np.ndarray:
     if not data:
         raise HTTPException(status_code=400, detail="empty image")
@@ -162,12 +176,15 @@ async def remove_background(
     file: UploadFile = File(...),
     screen_color: str | None = Form(None),
     seg_model: str | None = Form(None),
+    cross_check: str | None = Form(None),
 ):
     if _manager is None:
         raise HTTPException(status_code=503, detail="service not ready")
 
     normalized_color = _normalize_screen_color(screen_color)
     normalized_seg = _normalize_seg_model(seg_model) or _manager.default_seg_model
+    normalized_cc = _normalize_cross_check(cross_check)
+    effective_cc = normalized_cc if normalized_cc is not None else _manager.cross_check_enabled
 
     rgb = _decode_image(await file.read())
 
@@ -178,7 +195,10 @@ async def remove_background(
             result = await loop.run_in_executor(
                 None,
                 lambda: _manager.process(
-                    rgb, screen_color=normalized_color, seg_model=normalized_seg
+                    rgb,
+                    screen_color=normalized_color,
+                    seg_model=normalized_seg,
+                    cross_check=normalized_cc,
                 ),
             )
         except RuntimeError as exc:
@@ -200,5 +220,6 @@ async def remove_background(
         "X-BGFilter-Elapsed-Ms": str(elapsed_ms),
         "X-BGFilter-Seg-Model": normalized_seg,
         "X-BGFilter-Screen-Color": normalized_color if normalized_color else "auto",
+        "X-BGFilter-Cross-Check": "on" if effective_cc else "off",
     }
     return Response(content=png, media_type="image/png", headers=headers)