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/account/handlers.rs

@@ -194,8 +194,8 @@ pub async fn dashboard_stats(
     let today_row: DashboardTodayRow = sqlx::query_as(
         "SELECT
             (SELECT COUNT(*) FROM relay_tasks WHERE created_at >= $1 AND created_at < $2) as tasks_today,
-            COALESCE((SELECT SUM(input_tokens) FROM usage_records WHERE created_at >= $1 AND created_at < $2), 0) as tokens_input,
-            COALESCE((SELECT SUM(output_tokens) FROM usage_records WHERE created_at >= $1 AND created_at < $2), 0) as tokens_output"
+            COALESCE((SELECT SUM(input_tokens) FROM usage_records WHERE created_at >= $1 AND created_at < $2), 0)::bigint as tokens_input,
+            COALESCE((SELECT SUM(output_tokens) FROM usage_records WHERE created_at >= $1 AND created_at < $2), 0)::bigint as tokens_output"
     ).bind(&today_start).bind(&tomorrow_start).fetch_one(&state.db).await?;
 
     Ok(Json(serde_json::json!({