fix(llm-inference): switch serve phase from vLLM+bitsandbytes to llama.cpp+GGUF
bitsandbytes peak RAM ~54GB (bf16 load before quantize) — kills 40GB OptiPlex.
llama.cpp Q4_K_M GGUF loads pre-quantized: peak RAM ~15.5GB, fits cleanly.
Changes:
- serve.yml: build llama.cpp with CUDA, download Q4_K_M GGUF from bartowski,
disable vllm-serve, deploy llama-server.service
- llama-server.service.j2: OpenAI-compatible server on same port 8000,
--n-gpu-layers 99 (full GPU offload), --parallel 4, gemma chat template
- defaults: llm_gguf_dir, llm_gguf_path, llm_gpu_layers, llm_parallel_slots
- handlers: restart llama-server, vllm-serve failed_when=false (may not exist)
GGUF: bartowski/gemma-2-27b-it-Q4_K_M.gguf (15.5GB, 24GB VRAM fits w/ ~8GB headroom)
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