feat: DeepSeek V4 Flash primario, Big Pickle fallback

- deepseek-v4-flash su opencode/go come modello primario
- big-pickle su opencode/zen come fallback (gratuito)
- Fallback automatico su censura "User Safety" o errore HTTP
- Aggiornati .env, config.py, start.sh, docker-compose.yml

start.sh

@@ -19,6 +19,8 @@ export DCC_OLLAMA_MODEL="${DCC_OLLAMA_MODEL:-gemma2:9b}"
 export DCC_LOCAL_URL="${DCC_LOCAL_URL:-https://opencode.ai/zen/go/v1}"
 export DCC_LOCAL_KEY="${DCC_LOCAL_KEY:-}"
 export DCC_LOCAL_MODEL="${DCC_LOCAL_MODEL:-deepseek-v4-flash}"
+export DCC_LOCAL_FALLBACK_URL="${DCC_LOCAL_FALLBACK_URL:-https://opencode.ai/zen/v1}"
+export DCC_LOCAL_FALLBACK_MODEL="${DCC_LOCAL_FALLBACK_MODEL:-big-pickle}"
 
 # ── create data dir ────────────────────────────────────────────────
 mkdir -p data