Tokens now carry a keyed hash of the account's security stamp, and every
authenticated request compares it with the stored stamp (cached for 60 s,
evicted in-process on change). A password reset or change, a deactivation
and a deletion all rotate or remove the stamp, so tokens issued before them
get 401. Tokens without the claim get 401 too.
Storing the submitted full name in FirstName while leaving a seeded
LastName intact made an unchanged save project a duplicated name
(e.g. "Legacy Manager Manager"). Leave FirstName/LastName untouched when
the submitted name already matches the stored first+last projection, and
otherwise split the submitted name across FirstName/LastName so renames
round-trip cleanly.
CreateAsync checks FindByEmailAsync and then inserts, so two concurrent
creates with the same email can both pass the check and hit the unique
user-name index. That DbUpdateException was unhandled and surfaced as a
500. Catch it at the CreateAsync call and return the existing
"Email is already in use." message, matching the check-then-insert
converge pattern already used by SetOverrideCoreAsync.
