Tune gunicorn/Postgres/Redis for a memory-limited server

- gunicorn: 3 workers -> 2 (this workload isn't CPU-bound per
  DEPLOYMENT.md's own sizing), add --preload so workers share
  immutable memory via copy-on-write instead of each independently
  importing Django, add --max-requests so a worker that renders a
  WeasyPrint invoice doesn't carry that memory forever.
- Postgres: trim shared_buffers/max_connections from the image
  defaults (128MB/100), sized for a ~0.2GB dataset instead.
- Redis: cap with --maxmemory as a ceiling, not a saving.

compose.behind-proxy.yaml

@@ -43,6 +43,8 @@ services:
       POSTGRES_DB: ${POSTGRES_DB:-rosterchief}
       POSTGRES_USER: ${POSTGRES_USER:-rosterchief}
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?set a database password}
+    # See compose.yaml for why these are trimmed from the defaults.
+    command: ["postgres", "-c", "shared_buffers=64MB", "-c", "max_connections=20"]
     volumes:
       - pgdata:/var/lib/postgresql/data
     healthcheck:
@@ -54,7 +56,7 @@ services:
   redis:
     image: redis:7-alpine
     restart: unless-stopped
-    command: ["redis-server", "--save", "", "--appendonly", "no"]
+    command: ["redis-server", "--save", "", "--appendonly", "no", "--maxmemory", "32mb", "--maxmemory-policy", "allkeys-lru"]
 
 volumes:
   pgdata: