Support tool config in yaml

.env.example

@@ -11,9 +11,16 @@ PORT=8000
 # EXTERNAL_IP=1.2.3.4
 
 # Backend bridge (optional)
+# BACKEND_MODE=auto|http|disabled
+BACKEND_MODE=auto
 BACKEND_URL=http://127.0.0.1:8100
 BACKEND_TIMEOUT_SEC=10
+HISTORY_ENABLED=true
 HISTORY_DEFAULT_USER_ID=1
+HISTORY_QUEUE_MAX_SIZE=256
+HISTORY_RETRY_MAX_ATTEMPTS=2
+HISTORY_RETRY_BACKOFF_SEC=0.2
+HISTORY_FINALIZE_DRAIN_TIMEOUT_SEC=1.5
 
 # Audio
 SAMPLE_RATE=16000