First-class Auth Method abstraction for OAuth/SSO logins (WeChat, Google, etc.)

The Problem
Currently, Vaultwarden treats all logins as username + password. For OAuth/SSO logins (WeChat, Google, Apple, GitHub, etc.), users have to:
Leave the password field empty
Use custom fields with inconsistent naming​ (e.g., auth_method, login_type, provider...)
Manually remember which identity was used (critical when you have two Google accounts​ and forget which one was bound to a site)
This leads to data drift — some entries have the field, some don't, and the naming is never consistent. After 6 months, you open a site and have no idea whether to click "WeChat" or "Google", or which email to use.
Proposed Solution: First-Class auth_method Field
Add a native auth_method enum to the Login cipher type, making it a structured, validated field​ instead of a free-text custom field.