ds4: configurable CUDA env knobs (expert-cache reserve + free-form extra_env)

ds4-server exposes several CUDA tunables only via environment, not CLI flags.
By default ds4 reserves half the card for non-cache use and allocates the model
weight arena in 1792 MiB chunks — both starve / OOM the streaming expert cache
on small-weight MoE models served from SSD.

Pass an explicit env to ds4-server (Popen now sets env=) with:
  - expert_cache_reserve_gb: typed knob -> DS4_CUDA_STREAMING_EXPERT_CACHE_RESERVE_GB
    (0 = leave ds4's default).
  - extra_env: free-form KEY=VALUE passthrough for the rest, e.g.
    DS4_CUDA_WEIGHT_ARENA_CHUNK_MB=512 to shrink the weight-arena chunk so it
    fits a heap fragmented by the expert cache.

Both surfaced in Settings (config + admin GET/POST + UI), default to no-op so
behaviour is unchanged unless set.

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