Reports whether a candidate username is free to claim before the user commits to a full signup or migration flow. Intended for debounced calls as the user types. Returns available: false when the username already belongs to another AppUser; returns HTTP 400 (VALIDATIONFAILED) when the username fails the format rules (3–30 chars; starts with a lowercase letter; lowercase letters, digits, and internal . - separators). Note: the DB unique constraint enforced at signup/migrate time remains the source of truth — a username reported as available can still race with a concurrent claim.
Access: Public endpoint.
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Username failed format validation.
Too many username availability checks from this client. The error details include retryAfterSec (seconds until the window resets).