fix(saas): add ::bigint cast to all SUM() aggregates for PG NUMERIC compat

PostgreSQL SUM() on bigint returns NUMERIC, causing sqlx decode errors
when Rust expects i64/Option<i64>. Root cause: key_pool.rs
select_best_key() token_count SUM was missing ::bigint, causing
DATABASE_ERROR on every relay request.

Fixed in 4 files:
- relay/key_pool.rs: SUM(token_count) — root cause of relay failure
- relay/service.rs: SUM(remaining_rpm) in sort_candidates_by_quota
- account/handlers.rs: SUM(input/output_tokens) in dashboard stats
- workers/aggregate_usage.rs: SUM(input/output_tokens) in aggregation

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

@@ -648,7 +648,7 @@ pub async fn sort_candidates_by_quota(
         let quota_rows: Vec<(String, i64)> = match sqlx::query_as(
             r#"
             SELECT pk.provider_id,
-                   SUM(COALESCE(pk.max_rpm, 999999) - COALESCE(uw.request_count, 0)) AS remaining_rpm
+                   SUM(COALESCE(pk.max_rpm, 999999) - COALESCE(uw.request_count, 0))::bigint AS remaining_rpm
             FROM provider_keys pk
             LEFT JOIN key_usage_window uw ON pk.id = uw.key_id
                 AND uw.window_minute >= to_char(NOW() - INTERVAL '1 minute', 'YYYY-MM-DDTHH24:MI')