The passwordless migration (b330d77) removed the password-login route, leaving
concurrencyLimiter — its bcrypt concurrency cap — with no caller, and scattered
stale "local-password" / "change-password" references through the surviving auth
code's comments.
- Remove the dead concurrencyLimiter (type + newConcurrencyLimiter + acquire):
no caller, no struct field, no test. Reword the one streamLimiter doc that
contrasted against it.
- Realign comments in repo.go, pgrepo.go, session.go, util.go to the passwordless
reality: staff lookups feed email-OTP / passkey / setup redeem, not a password
compare; RevokeUserSessionsExcept and DeleteAllPasskeyCredentialsForUser are
retained (uncalled) for the P5 account-remediation path (#78); "local sessions"
no longer implies a password.
Comments and dead code only; no behavior change. Full WSL test tree green.
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