fix: make upload timeouts configurable, default to 1 hour (#942)

[alanbchristie]

Summary

Mirrors the upload-timeout fix from PR #943 (which targets staging) onto m2ms-lhs-rhs-unification so the long-lived feature branch picks it up directly without waiting for a merge from staging. Same defaults, same env vars.

Closes nothing — issue #942 stays linked via the staging PR.

The m2ms branch had enough divergence (no gunicorn.conf.py; gunicorn started inline in launch-stack.sh; older settings.py) that I re-applied the changes by hand rather than cherry-pick.

• nginx.conf / django_nginx.conf are now envsubst templates. All timeout directives derive from NGINX_TIMEOUT_S (default 3600s) rendered by launch-stack.sh before nginx -tq runs. envsubst is scoped to that single variable so nginx's own $vars (e.g. $http_host, regex anchors) pass through untouched.
• Gunicorn --timeout is now driven by STACK_GUNICORN_TIMEOUT_S (default 3600s), replacing the hard-coded 3000 in launch-stack.sh. (m2ms invokes gunicorn inline rather than via a config file, so this lives in the launch script — not in a gunicorn.conf.py.)
• Dockerfile pulls in gettext-base (provides envsubst) and stages the nginx configs as templates under /etc/nginx/templates/.
• Django DATA_UPLOAD_MAX_MEMORY_SIZE (default 18 GiB) and FILE_UPLOAD_MAX_MEMORY_SIZE (default 2.5 MiB, Django's own default made explicit) are now env-driven Django settings.

⚠️ Companion change required in fragalysis-stack-kubernetes

The Kubernetes NGINX ingress controller (v1.12.0) is configured in fragalysis-stack-kubernetes, not here. Its defaults (60s) will still produce 504s regardless of this PR. The ingress (or per-route annotations on the upload endpoint) must be raised in lockstep:

nginx.ingress.kubernetes.io/proxy-read-timeout:    "3600"
nginx.ingress.kubernetes.io/proxy-send-timeout:    "3600"
nginx.ingress.kubernetes.io/proxy-connect-timeout: "75"
nginx.ingress.kubernetes.io/proxy-body-size:       "0"

Test plan

• Python syntax check: python -m py_compile fragalysis/settings.py
• Bash syntax check: bash -n launch-stack.sh
• envsubst rendering simulated locally — both files render correctly, nginx $vars and regex anchors preserved
• pre-commit passes on the changed files
• docker compose up -d --build succeeds and nginx -tq (run inside launch-stack.sh) passes — please verify in a real container build
• docker compose exec backend cat /etc/nginx/nginx.conf | grep timeout shows 3600s on all seven directives; same for /etc/nginx/sites-available/default.conf
• Override test: set NGINX_TIMEOUT_S=120 and STACK_GUNICORN_TIMEOUT_S=120 in compose, restart, confirm rendered values / gunicorn process args change
• docker compose exec backend python manage.py shell -c "from django.conf import settings; print(settings.DATA_UPLOAD_MAX_MEMORY_SIZE, settings.FILE_UPLOAD_MAX_MEMORY_SIZE)" returns the expected ints
• End-to-end upload of a large file (>1 GiB) against a dev cluster with the ingress annotations above applied — confirms no 504

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