Parse CORS_HEADER as a list: * for all origins, or reflect matching
request Origin when multiple specific origins are configured. Add Vary:
Origin for the allowlist case. Update .env.example and CORS tests.
Add POST /v1/admin/users/{pubkey}/reset-username and
POST /v1/admin/users/reset-usernames to clear manual_username
and last_synced_at so nostr profile sync re-evaluates users.
Includes OpenAPI docs, audit actions, and tests.
