Bump TRL to 0.16.1 (adds GRPOTrainer); transformers 4.51.3, peft 0.14, accelerate 1.4, bnb 0.45.5

Overhaul trainer: TRL GRPO with env-backed reward, Qwen2.5-0.5B 4bit+LoRA, slim PyTorch CUDA base, heartbeat HTTP for HF Spaces health probe

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