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/services/audit_rule_seeds_v2.py

@@ -65,14 +65,14 @@ DDL_RULES: List[Dict[str, Any]] = [
         "message": "禁止执行 TRUNCATE TABLE 操作，数据不可回滚",
     },
     {
-        "name": "单表索引数量不得超过12个",
+        "name": "单表索引数量不得超过20个",
         "description": "单表索引数量过多会影响写入性能",
         "rule_type": "DDL",
         "severity": "warning",
         "db_type": "all",
         "pattern": None,
-        "value": "12",
-        "message": "单表索引数量超过12个，会影响写入性能",
+        "value": "20",
+        "message": "单表索引数量超过20个，会影响写入性能",
     },
     {
         "name": "单索引字段数不得超过8个",
@@ -95,24 +95,24 @@ DDL_RULES: List[Dict[str, Any]] = [
         "message": "禁止使用 FLOAT/DOUBLE 类型，建议使用 DECIMAL 避免精度丢失",
     },
     {
-        "name": "CHAR 类型长度不得超过10",
+        "name": "CHAR 类型长度不得超过255",
         "description": "CHAR 类型长度过大会浪费存储空间，超出请使用 VARCHAR",
         "rule_type": "DDL",
         "severity": "warning",
         "db_type": "all",
         "pattern": None,
-        "value": "10",
-        "message": "CHAR 类型长度不得超过10，超出请使用 VARCHAR",
+        "value": "255",
+        "message": "CHAR 类型长度不得超过255，超出请使用 VARCHAR",
     },
     {
-        "name": "表名长度不得超过45字符",
+        "name": "表名长度不得超过64字符",
         "description": "过长的表名不利于维护和使用",
         "rule_type": "DDL",
         "severity": "warning",
         "db_type": "all",
         "pattern": None,
-        "value": "45",
-        "message": "表名长度超过45个字符，请缩短表名",
+        "value": "64",
+        "message": "表名长度超过64个字符，请缩短表名",
     },
     {
         "name": "禁止外键约束",