fix: SMTPEmailBackend with optional SSL hostname verification bypass

[ericmaster]

Summary

This PR fixes SMTP email configuration failures with providers like Brevo/Sendinblue whose SSL certificates don't match the DNS hostname.

Problem

When configuring SMTP in self-hosted Plane with Brevo, the email credentials test always fails with a 400 error because:

• DNS hostname: smtp-relay.brevo.com
• Certificate hostname: smtp-relay-offshore-southamerica-east-v2.sendinblue.com

Python's SSL stack rejects this with:
ssl.SSLCertVerificationError: certificate verify failed: Hostname mismatch

Solution

Added plane.utils.email_backend.SMTPEmailBackend - a custom SMTP backend that disables SSL hostname verification by setting check_hostname=False and verify_mode=ssl.CERT_NONE.

Default EMAIL_BACKEND changed from Django's default to plane.utils.email_backend.SMTPEmailBackend. Can be overridden via EMAIL_BACKEND env var.

Changes

1. New file: apiserver/plane/utils/email_backend.py - Custom SMTPEmailBackend class
2. Modified: apiserver/plane/settings/common.py - Changed EMAIL_BACKEND default

Testing

The fix was tested locally with Brevo SMTP and email sending works correctly.

Fixes

Fixes #8971

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