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>

configs/gpu.yaml

@@ -73,3 +73,15 @@ foreground:
 
 despill:
   enabled: false
+
+server:
+  # Request concurrency (HTTP service only). cpu_workers = thread-pool size: >1 runs
+  # several requests at once, overlapping their CPU stages while the GPU forwards
+  # stay serialized -- keeps the GPU fed. gpu_concurrency = how many forwards may
+  # co-schedule on the SMs at once: 2 lifts GPU-bound throughput (batch-1 underfills
+  # the SMs) but roughly DOUBLES GPU activation memory.
+  #   * On a >=24 GB card (e.g. 4080 SUPER 32 GB): gpu_concurrency: 2 peaks ~19 GB.
+  #   * On a <=16 GB card: keep gpu_concurrency: 1 or it will OOM at 2048 cross-check.
+  # Env vars BGFILTER_CPU_WORKERS / BGFILTER_GPU_CONCURRENCY override these per box.
+  cpu_workers: 8
+  gpu_concurrency: 2