feat: 交易日9:31清空全部缓存，保证开盘后数据全新

- services/cache.py 新增 clear_all() 清空整个 cache 表
- daily_collector.py 新增 cache_cleanup_loop() 后台循环：精确 sleep 到最近一个
  工作日 9:31 清空缓存，跳过周末，出错1分钟重试
- main.py lifespan 启动清理任务，与采集任务一同优雅退出

与既有"非交易时段 TTL 截止到下次开盘前"的惰性过期形成双保险

Co-Authored-By: Claude <noreply@anthropic.com>

backend/main.py

@@ -8,7 +8,7 @@ from dotenv import load_dotenv
 
 from database import init_db
 from routes import stock, collections, shares, sectors, themes, core_stocks
-from services.daily_collector import collector_loop
+from services.daily_collector import collector_loop, cache_cleanup_loop
 
 load_dotenv()
 
@@ -17,14 +17,17 @@ load_dotenv()
 async def lifespan(app: FastAPI):
     init_db()
     collector_task = asyncio.create_task(collector_loop())
+    cache_cleanup_task = asyncio.create_task(cache_cleanup_loop())
     try:
         yield
     finally:
-        collector_task.cancel()
-        try:
-            await collector_task
-        except asyncio.CancelledError:
-            pass
+        for t in (collector_task, cache_cleanup_task):
+            t.cancel()
+        for t in (collector_task, cache_cleanup_task):
+            try:
+                await t
+            except asyncio.CancelledError:
+                pass
 
 
 app = FastAPI(title="AUV API", version="1.0.0", lifespan=lifespan)