mail: remember SMTP settings for each provider type

[t0ma5]

Summary

• Mail Settings only stored one live mailserver_* row, so saving Gmail erased Custom and there was no slot for a transactional provider.
• Server type is now Custom, Gmail, Brevo, SMTP2GO and Amazon SES. Each has its own mailserver_preset_{type} JSON preference. Factory host/port/encryption fill empty fields (no site credentials). Saving one type cannot wipe another: an empty posted slot for an inactive type is ignored.
• osc_sendMail() still reads the live mailserver_* preferences. The extra slots only park a second provider so you can switch and Save on the one that should send.
• Gmail still locks host/port to smtp.gmail.com / 465 / ssl. SES uses email-smtp.us-east-1.amazonaws.com as a region placeholder. Docs: docs/site/configure/mail-server.md.

This overlaps Mail Settings JS with #523 (test-email button). Merge that first if both land; rebase this one if mailserver.php conflicts.

Test plan

• php tests/mailserver-presets.php (normalize/factory/sanitize/merge: saving Brevo does not wipe stored SMTP2GO)
• Admin → Settings → Mail server: save Gmail, switch to Brevo, fill and save, switch back — Gmail username/password still there
• After Save on Brevo, site mail actually sends through Brevo (live mailserver_* is the active type)
• Custom and existing Gmail installs still load without a migration

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