feat(auth): merge mobile+email logins into one account via BizGaze person-id (#2)

Provisioning now keys the Biz Connect account on bz.bizgazeUserId (the same
value whether the person signs in with email or mobile) instead of the typed
identifier, so both logins resolve to a single contact. Legacy rows get the
person-id stamped on next login; an existing duplicate created under the same
identifier is folded in via a transactional users.mergeInto() that reassigns
all messages/memberships/reactions/votes/favorites/ownership to the survivor.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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