Rename OIDC strategy from :rauthy to :oidc, update callback path

- Rename AshAuthentication strategy from :oidc :rauthy to :oidc :oidc;
  generated actions are now register_with_oidc / sign_in_with_oidc.
- Update config keys (:rauthy → :oidc) in dev.exs and runtime.exs.
- Update default_redirect_uri to /auth/user/oidc/callback everywhere.
- Rename Mv.Accounts helper functions accordingly.
- Update Mv.Secrets, AuthController, link_oidc_account_live and all tests.
- Update docker-compose.prod.yml, .env.example, README and docs.

IMPORTANT: OIDC providers must be updated to use the new redirect URI
/auth/user/oidc/callback instead of /auth/user/rauthy/callback.

lib/mv_web/live/auth/link_oidc_account_live.ex

@@ -84,7 +84,7 @@ defmodule MvWeb.LinkOidcAccountLive do
           :info,
           dgettext("auth", "Account activated! Redirecting to complete sign-in...")
         )
-        |> Phoenix.LiveView.redirect(to: ~p"/auth/user/rauthy")
+        |> Phoenix.LiveView.redirect(to: ~p"/auth/user/oidc")
 
       {:error, error} ->
         Logger.warning(
@@ -223,7 +223,7 @@ defmodule MvWeb.LinkOidcAccountLive do
              "Your OIDC account has been successfully linked! Redirecting to complete sign-in..."
            )
          )
-         |> Phoenix.LiveView.redirect(to: ~p"/auth/user/rauthy")}
+         |> Phoenix.LiveView.redirect(to: ~p"/auth/user/oidc")}
 
       {:error, error} ->
         Logger.warning(