refactor(db): 移除无意义 actor，修复 history 多租户与模型

- 删除 entries_history / audit_log / secrets_history 的 actor 列及写入逻辑
- MCP secrets_history 透传当前 user_id
- Entry 增加 user_id，search 查询不再用伪 UUID
- 迁移：保留 users.api_key，从 api_keys 表回退时生成新明文 key 并删表
- 文档：audit_log auth 语义、API Key 存储说明

Made-with: Cursor

crates/secrets-core/src/service/audit_log.rs

@@ -8,7 +8,7 @@ pub async fn list_for_user(pool: &PgPool, user_id: Uuid, limit: i64) -> Result<V
     let limit = limit.clamp(1, 200);
 
     let rows = sqlx::query_as(
-        "SELECT id, user_id, action, namespace, kind, name, detail, actor, created_at \
+        "SELECT id, user_id, action, namespace, kind, name, detail, created_at \
          FROM audit_log \
          WHERE user_id = $1 \
          ORDER BY created_at DESC, id DESC \