stages/identification: passwordless flow button label is hardcoded to "Use a security key" #22042

Describe the bug

IdentificationStage.passwordless_flow is a generic mechanism — admins point it at any authentication flow. But the button text is hardcoded to "Use a security key", which does not match non-WebAuthn passwordless flows (e.g., email-based authentication).

To reproduce

1. Create an EmailStage flow for passwordless auth
2. Set passwordless_flow on an IdentificationStage to that flow
3. Visit the login page
4. Button says "Use a security key" regardless of what the flow does

Expected behavior
Admins should be able to configure the button label, or the label should reflect the flow's purpose.

Proposed solution
Add an optional passwordless_label field to IdentificationStage. When blank → existing behavior. When set → renders custom text.