perf: 翻译独立后台循环(1 篇/秒)+ Semaphore 1

之前 fetch_one_source 入库后立即调翻译(可能并发触发腾讯 TMT 限速)
改为独立 translation_loop 后台循环:
- 完全不和 RSS 抓取并行
- 1 篇/秒节拍(Semaphore 1 + sleep 1.0)
- 没活时空闲 5 秒再轮询
- pending/failed 都重试

backend/app/services/translation/service.py

@@ -31,7 +31,8 @@ class TranslationService:
     def __init__(self):
         self._tencent: BaseTranslator | None = None
         self._local: BaseTranslator | None = None
-        self._sem = asyncio.Semaphore(3)  # 并发限流
+        # 串行:1 个并发;避免触发腾讯 TMT 限速
+        self._sem = asyncio.Semaphore(1)
 
     def _primary(self) -> BaseTranslator:
         if self._tencent is None: