fix: destroy client socket on upstream mid-stream error

Prevents hanging connections when the upstream response stream errors
after headers have already been sent to the client.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api-gateway.cjs

@@ -410,6 +410,7 @@ async function handleRequest(req, res, pool) {
 
     upstreamRes.on("error", async (err) => {
       completed = true
+      res.destroy()
       const requestCompletedAt = new Date()
       const latencyMs = requestCompletedAt.getTime() - requestStartedAt.getTime()
       await failUsageEvent(pool, eventId, {
@@ -493,6 +494,7 @@ async function handleRequest(req, res, pool) {
     })
 
     upstreamRes.on("error", async (err) => {
+      res.destroy()
       const requestCompletedAt = new Date()
       const latencyMs = requestCompletedAt.getTime() - requestStartedAt.getTime()
       await failUsageEvent(pool, eventId, {