feat(backup): add independent backup panel, configurable policy, and rename support

Extract backup & restore into a standalone AccordionItem in Advanced settings.
Add configurable auto-backup interval (disabled/6h/12h/24h/48h/7d) and retention
count (3-50) via settings. Add per-backup rename with inline editing UI.

src-tauri/src/database/mod.rs

@@ -43,9 +43,6 @@ use std::sync::Mutex;
 
 // DAO 方法通过 impl Database 提供，无需额外导出
 
-/// 数据库备份保留数量
-const DB_BACKUP_RETAIN: usize = 10;
-
 /// 当前 Schema 版本号
 /// 每次修改表结构时递增，并在 schema.rs 中添加相应的迁移逻辑
 pub(crate) const SCHEMA_VERSION: i32 = 5;