Merge the mail.tm transport and email processing pipeline into main.
- Transport-agnostic email processor with mail.tm polling transport
- POST /mailgate webhook endpoint
- User/queue/ticket resolvers with stub user auto-creation
- Queue mail_alias column for email routing
- Migration renumbered to 0021 to follow main's 0019/0020
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
