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

**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.