Setup wizard: offer email one-time-code login for the admin account #695

Background

#565 / #690 let an existing admin promote an email (OTP) user, but the setup wizard can't create one. Its admin step offers only:

• email + password: CreatePasswordAdmin (auth/password.go) creates a password-provider user; nothing is sent to the email,
• GitHub: createAdminUser (wizard/wizard.go) via EnsureAdmin.

The OTP path (auth/otp.go) only calls UpsertUser, so it never bootstraps an admin. A site that wants OTP-only login today has to create a password admin, sign in by OTP, promote that account, and then demote the password one.

Proposal

Add a third option to the wizard's admin step: sign in with an emailed code.

• It is optional. The wizard keeps working with no mail server, and SMTP fields (smtp_host, smtp_from, etc.) appear only when this option is picked.
• The wizard sends a code to the entered address, and the operator enters it to finish. This also proves the SMTP settings work before the install completes.
• On success, create the email-provider user and its admin_users row in one transaction guarded by "no admin exists yet", like CreatePasswordAdmin (no ErrAdminExists race).
• Keep the existing setup-code gate (SetupUnlocked) in front of it.
• Persist the SMTP settings so login-page OTP is enabled afterwards.

Open questions

• Where the SMTP settings are stored: written to .env like the other wizard config, or kept as DB settings.
• Whether the README's "no mail server is needed" wording for the wizard should change to "needed only for the email-code option".

Follow-on from #565 / #690.