feat(auth): conversation_view preference, on by default

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

migrations/0018_conversation_view_preference.down.sql

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+ALTER TABLE mailboxes DROP COLUMN IF EXISTS conversation_view;

migrations/0018_conversation_view_preference.up.sql

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+-- Whether the webmail lists mail as conversations. On by default: it is what
+-- people coming from other mail programs expect, and it is one switch away.
+ALTER TABLE mailboxes ADD COLUMN conversation_view BOOLEAN NOT NULL DEFAULT true;