fix: streaming reliability, timeout, memory leaks, and retry improvements

- Fix streaming timeout: use AbortController for connection-only timeout
  instead of AbortSignal.timeout() that kills active SSE streams
- Fix fetchViaCurl: stream body via ReadableStream instead of buffering
  entire response in memory
- Fix JWT/aichat race condition: add Promise dedup to prevent concurrent
  bootstrap calls (10 requests = 1 bootstrap, not 10)
- Fix ACTIVE_READERS memory leak: auto-remove readers on stream completion
- Fix WebSocket backpressure: log warning when client buffer exceeds 1MB
- Add SSE heartbeat/keepalive: send ': keepalive' every 15s to prevent
  LB/proxy timeout during AI thinking
- Fix SSELineBuffer: graceful overflow handling (warn + discard instead
  of throwing error that crashes stream)
- Fix transformStream tight loop: yield to event loop after each chunk
  to prevent starvation
- Fix fetchViaCurl process cleanup: use SIGKILL + proper timeout cleanup
- Add retry on 502/504: retry transient server errors before returning
  to caller (both fetchWithRetry and fetchWithSessionRetry)

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     network_mode: host
     environment:
       HOST: "::"
-      PORT: "3000"
+      PORT: "4060"
       IPV6_SOURCES: "2001:df4:c140:1f::8,2001:df4:c140:1f::395,2001:df4:c140:1f::d9,2001:df4:c140:1f::db,2001:df4:c140:1f::dd,2001:df4:c140:1f::de,2001:df4:c140:1f::df,2001:df4:c140:1f:ffff:ffff:ffff:1,2001:df4:c140:1f:ffff:ffff:ffff:2,2001:df4:c140:1f:ffff:ffff:ffff:ff,2001:df4:c140:1f:ffff:ffff:ffff:100,2001:df4:c140:1f:ffff:ffff:ffff:f000,2001:df4:c140:1f:ffff:ffff:ffff:f0e6,2001:df4:c140:1f:ffff:ffff:ffff:f229,2001:df4:c140:1f:ffff:ffff:ffff:f340,2001:df4:c140:1f:ffff:ffff:ffff:f34b,2001:df4:c140:1f:ffff:ffff:ffff:f3ef,2001:df4:c140:1f:ffff:ffff:ffff:f4ba"
       RELAY_TIMEOUT_MS: "30000"
       RATE_LIMIT_MAX: "1000"