Registration: a failed verification email strands the account; resends to unknown addresses create records; no-SMTP silently auto-verifies #700

Three problems in registration and email verification.

1. A failed verification email strands the account. The account is saved before the email is sent, and nothing undoes the save when sending fails. The user is told to try again, but the username and email are now "already taken". The resend link can recover the account, but users rarely find it.
2. Resending verification to an address with no account creates a record. The resend endpoint needs no login. For an unknown address it stored a key-only record and emailed a verification link to whatever address was typed.
3. Without SMTP configured, new accounts were auto-verified with no warning. A typo'd, blank or unmounted EMAIL_SERVER_URL silently turned the site into open registration.

Fixed in #695:

• A failed send rolls back the new account. A failed resend restores the link the user already has.
• A resend to an unknown address does nothing, and answers the same way as a normal resend.
• Auto-verification now requires ALLOW_UNVERIFIED_REGISTRATION=true, and the server says at startup what registration will do.
• Sign-up failures now show a message instead of failing silently.