fix: prevent local loops when routing to the same address

[pplulee]

Summary

• reject Address Endpoints that would deliver mail back to the route's own incoming address
• apply the same check when adding extra endpoints or later changing an Address Endpoint address
• compare addresses case-insensitively so User@Example.com cannot bypass user@example.com

This stops a misconfigured self-forward (route user@example.com pointing at Address Endpoint user@example.com) from generating unbounded mail.

Test plan

• Create a route for user@example.com that forwards to Address Endpoint user@example.com and confirm save fails
• Repeat with mixed-case addresses (User@Example.com) and confirm it is also rejected
• Add the same address as an additional endpoint on an otherwise valid route and confirm save fails
• Change an existing Address Endpoint to the route's own address and confirm save fails
• Forward user@example.com to a different address and confirm save still succeeds

Made with Cursor