fix(billing): 防止余额计费持续透支

deploy/config.example.yaml

@@ -1027,6 +1027,10 @@ billing:
     # Number of requests to allow in half-open state
     # 半开状态允许通过的请求数
     half_open_requests: 3
+  # Conservative minimum balance required before forwarding balance-billed requests.
+  # Set to 0 to only require balance > 0.
+  # 余额计费请求转发前要求的保守最小余额；设为 0 则仅要求余额 > 0。
+  minimum_balance_reserve: 0.000001
   # Cache TTL (seconds) for per-user × per-platform quota records
   # 用户 × 平台 quota 缓存 TTL（秒），默认 86400=1天，覆盖典型 daily 窗口
   user_platform_quota_cache_ttl_seconds: 86400