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/workers/aggregate_usage.rs

@@ -56,8 +56,8 @@ async fn aggregate_single_account(db: &PgPool, account_id: &str) -> SaasResult<(
         .with_nanosecond(0).unwrap_or(now);
 
     let aggregated: Option<(i64, i64, i64)> = sqlx::query_as(
-        "SELECT COALESCE(SUM(input_tokens), 0), \
-                COALESCE(SUM(output_tokens), 0), \
+        "SELECT COALESCE(SUM(input_tokens), 0)::bigint, \
+                COALESCE(SUM(output_tokens), 0)::bigint, \
                 COUNT(*) \
          FROM usage_records \
          WHERE account_id = $1 AND created_at >= $2 AND status = 'success'"