chore: make compose ports configurable

[voidash]

What this changes

• The host ports in compose.yaml can be overridden with DB_PORT and API_PORT. The defaults are unchanged (5432, 3000). Before this, running the stack next to another Postgres or app on those ports meant editing the file.
• Documents that the migrate image also runs one-off tasks such as docker compose run --rm migrate bun run db:init.
• Fixes a stale Dockerfile comment that still mentioned the removed design CSS and fonts.

Closes #37

How to check it

The whole Docker path was tested from a fresh clone of main:

Step	Result
docker compose config with no apps/api/.env.local	fails with a clear "env file not found" (setup creates it; the README revamp documents this)
docker compose build	api image 341 MB, migrate image 2.07 GB
docker compose up -d	db healthy → migrate applies migrations and exits 0 → api healthy in ~30 s
/health, /en, /ne, /en/members, /en/issues, /v1/members	200
Security headers from #35	present
Container user	node (uid 1000), /app/storage writable
docker compose run --rm migrate bun run db:init on a fresh database	project initialised, 14 issues synced; /v1/project 404 → 200
docker compose run --rm migrate bun run sync:github	14 issues synced
down then up	data kept, migrations re-run harmlessly
DB_PORT=15432 API_PORT=13000 docker compose up -d	stack runs beside another stack that holds 5432 and 3000

Deliberately not changed: the refined database credentials and the volume names. Renaming them would start any existing deployment on an empty volume, so that needs a planned migration.

Checklist

• One issue per pull request
• Commits signed off
• No new dependency
• No secrets or personal data