docs: add STT override env vars to .env.example

.env.example

@@ -285,3 +285,14 @@ WANDB_API_KEY=
 
 # Groq API key (free tier — used for Whisper STT in voice mode)
 # GROQ_API_KEY=
+
+# =============================================================================
+# STT ADVANCED OVERRIDES (optional)
+# =============================================================================
+# Override default STT models per provider (normally set via stt.model in config.yaml)
+# STT_GROQ_MODEL=whisper-large-v3-turbo
+# STT_OPENAI_MODEL=whisper-1
+
+# Override STT provider endpoints (for proxies or self-hosted instances)
+# GROQ_BASE_URL=https://api.groq.com/openai/v1
+# STT_OPENAI_BASE_URL=https://api.openai.com/v1