fix(backend): API 文本输入加 max_length + 请求体大小中间件（CR-H9）

packages/shared/ww_shared/errors.py

@@ -12,6 +12,7 @@ class ErrorCode(StrEnum):
     CONFLICT_UNRESOLVED = "CONFLICT_UNRESOLVED"  # 未决冲突禁验收
     LLM_UNAVAILABLE = "LLM_UNAVAILABLE"  # 回退耗尽
     RATE_LIMITED = "RATE_LIMITED"
+    PAYLOAD_TOO_LARGE = "PAYLOAD_TOO_LARGE"  # 请求体超过大小上限（body-size 中间件）
     INTERNAL = "INTERNAL"
 
 
@@ -23,6 +24,7 @@ HTTP_STATUS: dict[ErrorCode, int] = {
     ErrorCode.CONFLICT_UNRESOLVED: 409,
     ErrorCode.LLM_UNAVAILABLE: 503,
     ErrorCode.RATE_LIMITED: 429,
+    ErrorCode.PAYLOAD_TOO_LARGE: 413,
     ErrorCode.INTERNAL: 500,
 }