fix(registrations): admin-API-key approve/reject writes a non-UUID reviewed_by and returns 500 #1234

internal/config/store_postgres_registrations.go:178,216 writes reg.ReviewedBy as-is into the reviewed_by UUID column. For a session authenticated with the admin API key, that value is the literal "admin-api-key", so approving or rejecting a registration through the admin key fails with SQLSTATE 22P02 and returns 500. Found in an independent review of #1141. It predates that PR and is unchanged by it.

Fix: store NULL (or a dedicated system marker in a separate column) when the reviewer is the admin key, and keep the audit trail in the existing log line. Add an integration test that approves and rejects via the admin key.