Multiple user accounts

[rokartur]

What

List accounts, add an account, remove an account. No roles, no organisations,
no email invitations.

Why

This is the only gap on the list that actually blocks usage rather than
inconveniencing it: a team today shares one admin password. That also quietly
breaks the audit log, which records the same email address for every action no
matter who performed it.

How

• apps/auth/src/server.ts:44 hard-closes sign-up/email once the first
  account exists. Do not reopen public sign-up. Add a route in the auth service
  that creates an account, callable only by the manager with an admin session.
• List and delete on the manager side against auth.db, which
  manager/internal/auth/auth.go:90 already reads.
• New page apps/web/src/routes/system.settings.users.tsx, structurally a clone
  of system.settings.tokens.tsx.
• Update docs/security.md.

Deliberately without RBAC

Every account is an admin. Roles land when somebody actually asks for
read-only access, and then it is one column, not a permission matrix.