docs: update .env.example to reflect all required env vars

All parameters are now required (no defaults in code), so .env.example
lists them all uncommented. Added NVIDIA_ENABLE_THINKING.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/.env.example

@@ -2,11 +2,12 @@
 NVIDIA_API_KEY=your_nvidia_api_key_here
 APP_PASSWORD=your_shared_password_here
 
-# Opzionali — i default sono in app/config.py
-# NVIDIA_BASE_URL=https://integrate.api.nvidia.com/v1
-# NVIDIA_MODEL=nvidia/nemotron-3-ultra-550b-a55b
-# NVIDIA_TIMEOUT_SECONDS=120
-# NVIDIA_MAX_TOKENS=16384
-# NVIDIA_TEMPERATURE=1.0
-# NVIDIA_TOP_P=0.95
-# NVIDIA_REASONING_BUDGET=2048
+# Parametri server
+NVIDIA_BASE_URL=https://integrate.api.nvidia.com/v1
+NVIDIA_MODEL=nvidia/nemotron-3-ultra-550b-a55b
+NVIDIA_TIMEOUT_SECONDS=120
+NVIDIA_MAX_TOKENS=16384
+NVIDIA_TEMPERATURE=1.0
+NVIDIA_TOP_P=0.95
+NVIDIA_REASONING_BUDGET=2048
+NVIDIA_ENABLE_THINKING=false