[优化] 重构数据库迁移逻辑，添加版本管理和验证功能

docs/design/development.md

@@ -181,6 +181,8 @@ tests/
 
 每次提升数据库版本号时，必须补充从上一版本升级到新版本的显式迁移方法。迁移方法必须包含升级后的校验逻辑；只有校验通过，才能写入新的数据库版本记录。
 
+v1-v7 视为历史初始基线，不再维护逐版本升级文件。从 v8 起，数据库迁移必须放在 `openflare_server/model/migrate` 目录中，并以目标版本命名文件，例如 `v16.go`。每个版本文件通过 `init()` 注册自己的迁移，当前数据库版本取已注册迁移的最大目标版本。不得为了整理文件而改变已发布 v8+ 迁移的语义。
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 新包启动后必须先检查数据库当前版本，再按顺序逐步升级到目标版本；禁止跳过中间升级步骤直接写目标版本。
 
 空库初始化可以直接建立当前版本结构，但初始化完成后仍必须执行同版本校验，并落库当前数据库版本。

docs/en/design/development.md

@@ -181,6 +181,8 @@ The database version number is defined in `openflare_server/model`, and it must
 
 Every time the database version number is upgraded, an explicit migration method from the previous version to the new version must be added. The migration method must contain validation logic after the upgrade; only when the validation passes can the new database version record be written.
 
+Versions 1 through 7 are treated as the historical initial baseline and no longer keep per-version upgrade files. Starting from v8, database migrations must be placed under `openflare_server/model/migrate` and named after the target version, such as `v16.go`. Each version file registers its migration through `init()`, and the current database version is derived from the highest registered target version. Do not change the semantics of released v8+ migrations merely to reorganize files.
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 After starting the new package, the database's current version must be checked first, and then upgraded step by step in order to the target version; skipping intermediate upgrade steps to directly write the target version is prohibited.
 
 An empty database initialization can directly establish the current version structure, but the same-version validation must still be executed after the initialization is completed, and the current database version must be persisted.