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)

ansible/roles/llm-inference/handlers/main.yml

@@ -13,6 +13,13 @@
     name: vllm-serve
     state: restarted
   listen: "restart vllm-serve"
+  failed_when: false
+
+- name: Restart llama-server
+  ansible.builtin.systemd:
+    name: llama-server
+    state: restarted
+  listen: "restart llama-server"
 
 - name: Restart Hermes on carousel
   ansible.builtin.systemd: