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;