Update ISO installer tiers and backend deployment

ansible/roles/vllm/defaults/main.yml

@@ -0,0 +1,7 @@
+---
+# vLLM role defaults — overridden per-tier in the tier playbook vars block
+vllm_model: "meta-llama/Meta-Llama-3.1-8B-Instruct"
+vllm_tensor_parallel: 1
+vllm_gpu_memory_util: "0.70"
+vllm_max_model_len: 8192
+vllm_quantization: ""              # blank = full precision; set to "awq" for 4-bit