fix: 修复连接池计数和任务调度器默认值问题

1. db_pool.py - 修复连接计数不一致问题
   - 将 _created_connections 递增移到 put() 成功之后
   - 确保 Full 异常和创建异常时正确关闭连接
   - 避免计数器永久偏高

2. services/tasks.py - 统一 _running_by_user 默认值
   - 将减少计数时的默认值从 1 改为 0
   - 与增加计数时的默认值保持一致
   - 添加注释说明

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

db_pool.py

@@ -109,17 +109,26 @@ class ConnectionPool:
             with self._lock:
                 # 双重检查：确保池确实需要补充
                 if self._pool.qsize() < self.pool_size:
+                    new_conn = None
                     try:
                         new_conn = self._create_connection()
-                        self._created_connections += 1
                         self._pool.put(new_conn, block=False)
+                        # 只有成功放入池后才增加计数
+                        self._created_connections += 1
                     except Full:
                         # 在获取锁期间池被填满了，关闭新建的连接
-                        try:
-                            new_conn.close()
-                        except Exception:
-                            pass
+                        if new_conn:
+                            try:
+                                new_conn.close()
+                            except Exception:
+                                pass
                     except Exception as create_error:
+                        # 创建连接失败，确保关闭已创建的连接
+                        if new_conn:
+                            try:
+                                new_conn.close()
+                            except Exception:
+                                pass
                         print(f"重建连接失败: {create_error}")
 
     def close_all(self):