update llm.txt

backend/llm.txt

@@ -21,6 +21,8 @@ This file condenses `backend/docs/dev/http_api.md` + `backend/docs/dev/model.md`
 - DO keep Swagger annotations consistent with actual Fiber route paths (including `:param`).
 - MUST: route path parameter placeholders MUST be `camelCase` (e.g. `:tenantCode`), never `snake_case` (e.g. `:tenant_code`).
 - MUST: when creating/generating Go `struct` definitions (DTOs/requests/responses/etc.), add detailed per-field comments describing meaning, usage scenario, and validation/usage rules (do not rely on “self-explanatory” names).
+- MUST: business code comments MUST be written in Chinese (中文注释), to keep review/maintenance consistent across the team.
+- MUST: in `backend/app/services`, add Chinese comments at key steps to explain business intent and invariants (e.g., 事务边界、幂等语义、余额冻结/扣减/回滚、权限/前置条件校验点), avoid “what the code does” boilerplate.
 
 ---