修复高优先级安全和稳定性问题

1. app.py:
   - 添加user_semaphores_lock线程锁，修复get_user_semaphore竞态条件
   - 将9处裸except:改为except Exception:或具体异常类型

2. database.py:
   - 将3处裸except:改为except Exception:

3. email_service.py:
   - 添加_smtp_config_lock线程锁
   - 修复daily_sent计数竞态条件：获取配置时预增，失败时回退
   - _get_available_smtp_config和_get_next_available_smtp_config使用锁保护

🤖 Generated with Claude Code

database.py

@@ -837,7 +837,7 @@ def update_user_email(user_id, email, verified=False):
         # 先检查email_verified字段是否存在，不存在则添加
         try:
             cursor.execute('SELECT email_verified FROM users LIMIT 1')
-        except:
+        except Exception:
             cursor.execute('ALTER TABLE users ADD COLUMN email_verified INTEGER DEFAULT 0')
             conn.commit()
 
@@ -857,7 +857,7 @@ def update_user_email_notify(user_id, enabled):
         # 先检查字段是否存在
         try:
             cursor.execute('SELECT email_notify_enabled FROM users LIMIT 1')
-        except:
+        except Exception:
             cursor.execute('ALTER TABLE users ADD COLUMN email_notify_enabled INTEGER DEFAULT 1')
             conn.commit()
 
@@ -881,7 +881,7 @@ def get_user_email_notify(user_id):
             if row is None:
                 return True
             return bool(row[0]) if row[0] is not None else True
-        except:
+        except Exception:
             return True  # 字段不存在时默认开启