refactor(shared): remove /api prefix on backend URLs

[tsa96]

The new production deploy uses api.momentum-mod.org / staging-api.momentum-mod.org, so /api/ is redundant. In dev I don't think the prefix is particularly helpful either.

Checks

• [x] I have included/updated tests where applicable (see Testing)
• [x] I have ran nx run db:create-migration and committed the migration if I've made DB schema changes
• [x] I have followed semantic commit messages e.g. feat: Add foo, chore: Update bar, etc...
• [x] My branch has a clear history of changes that can be easy to follow when being reviewed commit-by-commit
• [x] My branch is functionally complete; the only changes to be done will be those potentially requested in code review
• [x] All changes requested in review have been fixuped into my original commits