feat(security): 启动时自动迁移并持久化JWT密钥

- 新增 security_secrets 表及 Ent schema 用于存储系统级密钥
- 启动阶段支持无 jwt.secret 配置并在数据库中自动生成持久化
- 在 Ent 初始化后补齐密钥并执行完整配置校验
- 增加并发与异常分支单元测试，覆盖密钥引导核心路径

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/ent/predicate/predicate.go

@@ -39,6 +39,9 @@ type Proxy func(*sql.Selector)
 // RedeemCode is the predicate function for redeemcode builders.
 type RedeemCode func(*sql.Selector)
 
+// SecuritySecret is the predicate function for securitysecret builders.
+type SecuritySecret func(*sql.Selector)
+
 // Setting is the predicate function for setting builders.
 type Setting func(*sql.Selector)