feat: 审核规则持久化到DB + 全面配置去硬编码化

- AuditRule模型: 新增code/config_json字段，支持规则编码和阈值参数持久化
- audit_engine_v2: _load_rules从DB读config_json覆盖默认config
- audit_rule_sync: 启动时自动从_RULE_REGISTRY同步规则到DB(239条)
- settings API: update后刷新query_service配置缓存
- query_service: MAX_ROWS/QUERY_TIMEOUT/MAX_QUERY_LENGTH/MAX_RESULT_MEMORY改为动态读取system_settings
- ticket_executor: SQL_TIMEOUT/REDIS_TIMEOUT/MONGO_TIMEOUT改为动态获取
- settings_seeds: 新增executor分组(sql/redis/mongo超时) + query_result_max_memory_mb
- audit_rule endpoint: 移除无效的AuditEngine缓存清理（每次audit是新实例）
- 测试: 1057 passed（修复所有新引入失败，17个历史遗留）

backend/app/models/audit_rule.py

@@ -7,6 +7,7 @@ class AuditRule(Base):
     __tablename__ = "audit_rules"
 
     id = Column(Integer, primary_key=True, autoincrement=True)
+    code = Column(String(50), nullable=True, unique=True, comment="规则编码，如 MYSQL-DDL-001")
     name = Column(String(100), nullable=False, unique=True)
     description = Column(Text, nullable=True)
     rule_type = Column(String(50), nullable=False)  # syntax / performance / safety / naming / DDL / DML
@@ -14,6 +15,7 @@ class AuditRule(Base):
     db_type = Column(String(20), default="mysql")  # mysql / postgres / redis / all
     pattern = Column(Text, nullable=True)  # regex or keyword pattern
     value = Column(String(255), nullable=True)  # 规则参数值，如 DDLMaxKeyParts=8 → value="8"
+    config_json = Column(Text, nullable=True, comment="规则配置JSON，如 {\"max_index\":20,\"enabled\":true}")
     is_enabled = Column(Boolean, default=True)
     message = Column(String(500), nullable=False)  # human-readable message
     created_at = Column(DateTime, server_default=func.now())