feat(key-pool): add LRU sorting via last_used_at column

- Add migration to add last_used_at TIMESTAMPTZ column to provider_keys
- Update select_best_key() SQL to sort by last_used_at ASC NULLS FIRST
- Update record_key_usage() to set last_used_at = NOW() on each use
- Bump SCHEMA_VERSION to 10

crates/zclaw-saas/src/db.rs

@@ -4,7 +4,7 @@ use sqlx::postgres::PgPoolOptions;
 use sqlx::PgPool;
 use crate::error::SaasResult;
 
-const SCHEMA_VERSION: i32 = 9;
+const SCHEMA_VERSION: i32 = 10;
 
 /// 初始化数据库
 pub async fn init_db(database_url: &str) -> SaasResult<PgPool> {