feat(backend): add structured audit logging for admin and auth actions

[Banx17]

Closes #824

[ogazboiz]

heads up: main's ci was broken (the Backend CI and Backend Docker Image CI jobs) until the fixes in #969 and #974 just landed, so the red backend/docker checks on this pr are almost certainly stale, they ran against the broken main. please rebase to re-test against the now-green main: git fetch origin && git rebase origin/main && git push --force-with-lease. once it's green i'll review and merge. (if a non-backend check like Frontend CI is still red after the rebase, that part is a real issue worth a look, since frontend ci was passing on main.)

[ogazboiz]

the structured audit logging looks clean (and it omits secrets, good). the red is infra noise, the Docker CI 'could not find prisma schema' plus a frontend parse error in create/page.tsx which your pr doesn't touch (fixed by #943). stale base, rebase: git fetch origin && git rebase origin/main && git push --force-with-lease.