Push GetSandboxMetricPoints time filter into SQL

The query was fetching all rows for a (sandbox_id, tier) pair and
filtering by timestamp in Go. For repeatedly-paused sandboxes the
24h tier can accumulate up to 30 days of data, causing up to 120x
over-fetching for a 6h range request.

Add AND ts >= $3 to the query so Postgres filters on the primary key
(sandbox_id, tier, ts) directly. Drop the redundant Go-side loop.

db/queries/metrics.sql

@@ -35,7 +35,7 @@ ON CONFLICT (sandbox_id, tier, ts) DO NOTHING;
 -- name: GetSandboxMetricPoints :many
 SELECT ts, cpu_pct, mem_bytes, disk_bytes
 FROM sandbox_metric_points
-WHERE sandbox_id = $1 AND tier = $2
+WHERE sandbox_id = $1 AND tier = $2 AND ts >= $3
 ORDER BY ts ASC;
 
 -- name: DeleteSandboxMetricPoints :exec