Reorganizes email storage: the DB moves from DATA_PATH/<user>/emails.db to
DATA_PATH/<user>/email_accounts/<accountEmail>/emails.db, with a shared
email_accounts/attachment_cache/ (was Gmail/emails/attachments). openEmailDb now
takes (owner, account); a new openUserEmailDb(owner, userId) resolves the user's
configured account (first enabled) for read paths. Threads the account through
email.ts, accounts, resync, queue sync, channel handlers, and the email_db MCP
tool path. Drops the dead getUserEmailDir helper.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>