fix SMTP VRFY returning 502 by migrating to emersion/go-smtp

[Mzack9999]

Summary

• Replace git.mills.io/prologic/smtpd with github.com/emersion/go-smtp, which responds to VRFY with 252 instead of 502 Command not implemented
• Clients such as curl that run VRFY before MAIL/DATA can now continue the SMTP session instead of aborting
• Add smtp_backend.go implementing the emersion Backend/Session while keeping existing interaction storage behavior on DATA

Closes #991

Test plan

• go test ./pkg/server/ -run TestSMTPVRFY -v
• go test ./pkg/server/
• go build ./...

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