docs: add PPLX_MODEL to .env.example

.env.example

@@ -9,6 +9,8 @@ XIMILAR_API_KEY=your-ximilar-api-key-here
 # Perplexity API Key (AI deck research & generation)
 # Get yours at: https://www.perplexity.ai/settings/api
 PPLX_API_KEY=your-perplexity-api-key-here
+# Perplexity model for deck research & generation (sonar / sonar-pro / sonar-deep-research)
+PPLX_MODEL=sonar-pro
 
 # CORS allowed origins (comma-separated)
 # Default: http://localhost:5173,http://localhost:8080