auto_slug_from_email maps an email's local-part to a schema-valid base
slug (lowercase, [+_.]→'-', alphanumeric-and-hyphen, clamp 32 chars,
fallback to user-{token_hex(4)} for pathological inputs).
next_available_slug walks the {base, base-2, base-3, ...} sequence
against a caller-supplied async exists_check until it lands on a free
slot, truncating base when the suffix would push past 32 chars.
Pulled forward of T4.8/T4.9 because both /auth/initialize and
/auth/register need it. Lives in app.gateway.auth (not persistence)
since "email → slug" is a registration-time concern.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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