Remove the oauth2 hex package and the legacy auth_providers modules

[elias-ba]

The oauth2 hex package is only used today by an older OIDC login implementation under lib/lightning/auth_providers/. The credential OAuth flow has been on Tesla (via OauthHTTPClient) for a while. This ticket retires the package once the audit in #4702 confirms no one's using the per-instance OIDC login in production.

What to remove

• The oauth2 dependency in mix.exs.
• Lightning.AuthProviders.Handler, Common, Google, Salesforce, OAuthBehaviour. The last three already have zero callers in lib/ or test/.
• The cache pieces (Store, CacheWarmer) that warm a single row from the auth_providers table.
• The auth_providers DB table.
• The login portion of LightningWeb.OidcController (the show/2 action and the first clause of new/2), the auth_handler_url/0 button on /users/log_in, and the admin LiveView at /settings/authentication.

What to keep

The other two clauses of OidcController.new/2 handle the popup callback for the credential OAuth flow at /authenticate/callback. They're alive and OauthHTTPClient depends on them. Move them to a clearer module before deleting OidcController.

Order

1. Land Decide what to do about the existing per-instance identity provider in production #4702's audit answer.
2. Move the popup clauses to a new controller, update RouteHelpers.oidc_callback_url/0.
3. Delete the legacy modules, drop the table.
4. Remove the oauth2 dep.