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/migrations/20260401000001_provider_keys_last_used.sql

@@ -0,0 +1,3 @@
+-- 20260401000001_provider_keys_last_used.sql
+-- Key Pool LRU: 记录每个 key 最后使用时间
+ALTER TABLE provider_keys ADD COLUMN IF NOT EXISTS last_used_at TIMESTAMPTZ;

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> {

crates/zclaw-saas/src/relay/key_pool.rs

@@ -51,7 +51,7 @@ pub async fn select_best_key(db: &PgPool, provider_id: &str, enc_key: &[u8; 32])
              LEFT JOIN key_usage_window uw ON pk.id = uw.key_id AND uw.window_minute = $1
              WHERE pk.provider_id = $2 AND pk.is_active = TRUE
                AND (pk.cooldown_until IS NULL OR pk.cooldown_until <= $3)
-             ORDER BY pk.priority ASC"
+             ORDER BY pk.priority ASC, pk.last_used_at ASC NULLS FIRST"
         ).bind(&current_minute).bind(provider_id).bind(&now).fetch_all(db).await?;
 
     for (id, key_value, priority, max_rpm, max_tpm, quota_reset_interval, req_count, token_count) in &rows {
@@ -165,6 +165,12 @@ pub async fn record_key_usage(
     .bind(tokens).bind(&chrono::Utc::now().to_rfc3339()).bind(key_id)
     .execute(db).await?;
 
+    // 更新最后使用时间 (LRU 排序依据)
+    sqlx::query("UPDATE provider_keys SET last_used_at = NOW() WHERE id = $1")
+        .bind(key_id)
+        .execute(db)
+        .await?;
+
     Ok(())
 }