feat: 审核规则模版+设置页面整合+去掉角色切换demo

后端:
- AuditRule 新增 value 字段, rule_type 扩展 DDL/DML
- MySQL/PG/Redis 预设规则种子接口 POST /audit-rules/seed/{db_type}
- 公告管理 CRUD (announcements)
- 消息推送 CRUD+测试 (notifications: 钉钉/飞书/邮件)
- LDAP 配置 CRUD+测试连接 (ldap-config)
- 164 tests passing (+31 新增)

前端:
- AuditRulesPage 重写: DB类型Tab+DDL/DML分组+参数编辑
- SettingsPage: 系统设置主页(审核规则/公告/推送/LDAP)
- NotificationPage: 钉钉/飞书/SMTP配置
- LdapConfigPage: LDAP设置+测试连接
- AnnouncementPage: 公告Markdown编辑
- 去掉左下角角色切换demo
- 侧边栏合并为: 系统设置(含审核规则+公告+推送+LDAP)

backend/app/models/notification.py

@@ -0,0 +1,15 @@
+from sqlalchemy import Column, Integer, String, Text, Boolean, DateTime, func
+
+from app.core.database import Base
+
+
+class NotificationChannel(Base):
+    __tablename__ = "notification_channels"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    name = Column(String(100), nullable=False)
+    channel_type = Column(String(30), nullable=False)  # dingtalk / webhook_feishu / email
+    config = Column(Text, nullable=False)  # JSON string
+    is_enabled = Column(Boolean, default=True)
+    created_at = Column(DateTime, server_default=func.now())
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now())