Cross-crate authn/authz data types for Phase 3.5. The Principal struct
is the resolved caller identity that auth_middleware will produce for
both cookie sessions and bearer API keys; the role/scope enums mirror
the DB CHECK constraints from migration 0006 and round-trip through
their stable string forms.
UserId is a type alias for AdminUserId — the auth layer treats an
admin row as the principal identity, so the alias avoids a rename of
the existing id type.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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