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.

test/accounts/user_email_sync_test.exs

@@ -206,7 +206,7 @@ defmodule Mv.Accounts.UserEmailSyncTest do
       # Simulate OIDC registration
       {:ok, user} =
         Mv.Accounts.User
-        |> Ash.Changeset.for_create(:register_with_rauthy, %{
+        |> Ash.Changeset.for_create(:register_with_oidc, %{
           user_info: user_info,
           oauth_tokens: oauth_tokens
         })