fix(prod): bump nginx mem_limit 64M → 256M to stop OOM during backend recreates

Root-caused task #103: nginx Exited(137) during backend
docker-compose --build && up -d flows. The 64M limit was tiny for
nginx + WebSocket upgrade buffering + gzip + SSL termination during
reconnection storms (every WS-connected client tries to reconnect
through nginx simultaneously when backend recreates). Docker
OOM-killed nginx with SIGKILL; restart: unless-stopped recovered it
but with a 5-15s outage window.

Fix: bump to 256M. EC2 has 3.7GiB total with 2.3GiB available.
Post-mortem at docs/internal/nginx-103-postmortem.md.
Applied via docker-compose up -d nginx after pull (using yesterday's
env-loading fix in deploy-prod.sh).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: kenneives

docker-compose.prod.yml

@@ -26,7 +26,11 @@ services:
     deploy:
       resources:
         limits:
-          memory: 64M
+          # 256M (bumped from 64M 2026-05-28 per task #103 post-mortem).
+          # nginx + WS upgrade buffering + gzip + SSL during backend recreates
+          # was hitting the 64M limit and getting OOM-killed (exit 137).
+          # EC2 has 2.3GiB available; 192MiB bump is negligible.
+          memory: 256M
 
   backend:
     build: