Lays the groundwork for per-user accounts: CreateUser/GetUserBySub/
ListUsers plus ProvisionUser, which seeds a brand-new user's profile,
default workout-kind taxonomy, and sync state in one transaction. Depends
on later tasks scoping GetProfile/ListWorkoutKinds/GetSyncState to compile
and pass -- expected or committing to a shared branch.
Christophe Vila
36bc651d66