The report endpoint returns 504 — and the server logs a 200 later

• GET /reports/annual returns 504 Gateway Timeout after exactly 60 s.
• The application log shows the same request completing with 200 about 75 s after it started.
• Users click "retry", which makes everything slower for everyone.
• Small reports work; only the annual report fails.

$ curl -o /dev/null -sw 'code=%{http_code} connect=%{time_connect}s ttfb=%{time_starttransfer}s total=%{time_total}s\n' \
    https://api.example.com/reports/annual
code=504 connect=0.021s ttfb=60.004s total=60.004s

# load balancer access log
2026-08-25T10:02:14Z ELB  203.0.113.9:51230  10.0.3.14:8080  0.001 60.000 -1  504  "GET /reports/annual"
#                         client            target        req  target  resp  status
#                                                         time time    time

# application log (same request id)
10:02:14.021 INFO  req=7f3a start GET /reports/annual
10:03:29.410 INFO  req=7f3a done 200 in 75389 ms   # nobody is listening anymore

# LB configuration
idle_timeout.timeout_seconds = 60