Was only ever settable via .env - now follows the same pattern as
Cloudflare Turnstile: SystemSecretKey.RESEND_API_KEY + a META entry
covers storage/encryption/frontend rendering automatically (the
Settings UI's Server secrets box is fully data-driven off this list).
Wired the register/login/resend-verification/request-password-reset
route handlers to use get_effective_settings so an admin-set key
actually reaches the emails those flows send, not just .env's value.
Co-Authored-By: Claude Sonnet 5 <[email protected]>