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/models.rs

@@ -9,6 +9,7 @@ use uuid::Uuid;
 #[derive(Debug, Serialize, Deserialize, sqlx::FromRow)]
 pub struct Entry {
     pub id: Uuid,
+    pub user_id: Option<Uuid>,
     pub namespace: String,
     pub kind: String,
     pub name: String,
@@ -184,7 +185,6 @@ pub struct AuditLogEntry {
     pub kind: String,
     pub name: String,
     pub detail: Value,
-    pub actor: String,
     pub created_at: DateTime<Utc>,
 }