[INFRA] feat: respect pre-set port env vars when initializing worktree ports

[ntilwalli]

Details

init_worktree_ports() in scripts/worktree-utils.sh unconditionally overwrites NGINX_PORT (and every sibling port) with BASE_* + PORT_OFFSET, so callers can't pin a single port via env (e.g. when 5173 is already taken on the host by another local app that can't move). The compose file already plumbs NGINX_PORT through to the frontend container's port mapping, healthcheck, and nginx config — only the unconditional overwrite blocks it from working.

This PR switches each port assignment in init_worktree_ports to the ${VAR:-default} idiom so pre-set env vars win while preserving the worktree-offset behavior for everything else. It also:

• Fixes OPIK_REVERSE_PROXY_URL's default in docker-compose.yaml to follow ${NGINX_PORT:-5173} instead of hardcoding 5173, so the python-backend can still reach the frontend on a non-default port.
• Adds a short "Changing the Frontend Port" section to deployment/docker-compose/README.md documenting NGINX_PORT (and pointing at OPIK_PORT_OFFSET for the all-ports-shifted case).

Change checklist

• User facing
• Documentation update

Issues

• Resolves #
• OPIK-

AI-WATERMARK

AI-WATERMARK: yes

• Tools: Claude Code
• Model(s): Claude Opus 4.7
• Scope: Code edits in scripts/worktree-utils.sh and deployment/docker-compose/docker-compose.yaml, README addition in deployment/docker-compose/README.md, commit message, and this PR description.
• Human verification: Author dogfooded the change end-to-end with NGINX_PORT=5293 ./opik.sh against this branch — confirmed boot banner shows http://localhost:5293, frontend container binds 0.0.0.0:5293->5293/tcp, UI returns 200, /api/is-alive/ver and /api/v1/private/projects return 200 through the reverse proxy, and python-backend's rendered OPIK_REVERSE_PROXY_URL resolves to http://frontend:5293/api.

Testing

• bash -n scripts/worktree-utils.sh — syntax check.
• Local docker-compose run with NGINX_PORT=5293 ./opik.sh: confirmed docker ps shows 0.0.0.0:5293->5293/tcp on frontend-1, the boot banner prints http://localhost:5293, the UI loads, /api/is-alive/ver returns 200 through the proxy, and python-backend's OPIK_REVERSE_PROXY_URL env var resolves to http://frontend:5293/api (verified via docker inspect).
• docker compose --profile opik config | grep OPIK_REVERSE_PROXY_URL — confirmed compose substitution renders correctly with both NGINX_PORT set and unset.
• Default invocation (./opik.sh with no env overrides) is unchanged — ${VAR:-…} falls through to the existing BASE_* + PORT_OFFSET expression, and OPIK_REVERSE_PROXY_URL resolves to http://frontend:5173/api as before.
• Multi-worktree use case is preserved: when no port var is pre-set, the offset still applies as today.

Documentation

Added a "Changing the Frontend Port" subsection to deployment/docker-compose/README.md covering NGINX_PORT and pointing at OPIK_PORT_OFFSET as the all-ports alternative.