Make the pipelining knobs config-file settable (server: section)

CPU_WORKERS / GPU_CONCURRENCY were env-only. Add a `server:` config
section (server.cpu_workers, server.gpu_concurrency) so a lane's
concurrency lives in its yaml alongside device/precision, matching how
model.device already works. Precedence mirrors BGFILTER_DEVICE: config is
the default, the env var overrides per box.

The pool + admission semaphore now build in lifespan (after the config
loads) instead of at import, and PipelineManager reads gpu_concurrency
from the settings. configs/gpu.yaml ships 8 / 2 (its target is the 32 GB
GPU boxes) with an inline VRAM caveat: gpu_concurrency>1 must fit
weights + N × forward, so keep it 1 on ≤16 GB cards.

Verified on a 4080 SUPER: startup logs cpu_workers=8 gpu_concurrency=2
from gpu.yaml with no env set, request returns a valid RGBA.

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

bgfilter/config.py

@@ -13,6 +13,7 @@ from .settings import (
     ModelSettings,
     PipelineSettings,
     SegmentationSettings,
+    ServerSettings,
     TrimapSettings,
 )
 
@@ -49,7 +50,7 @@ def _update_dataclass(instance: T, values: dict[str, Any] | None) -> T:
 def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
     data = dict(data)
     screen_color = data.pop("screen_color", None)
-    allowed_sections = {"chroma", "trimap", "alpha_post", "cross_check", "foreground", "despill", "model", "segmentation"}
+    allowed_sections = {"chroma", "trimap", "alpha_post", "cross_check", "foreground", "despill", "model", "segmentation", "server"}
     unknown_sections = sorted(set(data) - allowed_sections)
     if unknown_sections:
         raise ValueError(f"Unknown config section(s): {', '.join(unknown_sections)}")
@@ -63,6 +64,7 @@ def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
         despill=_update_dataclass(DespillSettings(), data.get("despill")),
         model=_update_dataclass(ModelSettings(), data.get("model")),
         segmentation=_update_dataclass(SegmentationSettings(), data.get("segmentation")),
+        server=_update_dataclass(ServerSettings(), data.get("server")),
         screen_color=screen_color,
     )
 
@@ -141,5 +143,6 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
         despill=_update_dataclass(despill, despill_updates),
         model=_update_dataclass(model, model_updates),
         segmentation=_update_dataclass(segmentation, seg_updates),
+        server=settings.server,
         screen_color=screen_color,
     )