merge: inbound email processing from worktree/ageless-shale

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>

drizzle/migrations/0021_romantic_captain_midlands.sql

@@ -0,0 +1 @@
+ALTER TABLE "queues" ADD COLUMN "mail_alias" text;