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/mv/oidc_role_sync_test.exs

@@ -104,8 +104,8 @@ defmodule Mv.OidcRoleSyncTest do
     end
   end
 
-  # B3: Role sync after registration is implemented via after_action in register_with_rauthy.
-  # Full integration tests (create_register_with_rauthy + assert role) are skipped: when the
+  # B3: Role sync after registration is implemented via after_action in register_with_oidc.
+  # Full integration tests (create_register_with_oidc + assert role) are skipped: when the
   # nested Ash.update! runs inside the create's after_action, authorization may evaluate in
   # the create context so set_role_from_oidc_sync bypass does not apply. Sync logic is covered
   # by the apply_admin_role_from_user_info tests above. B4 sign-in sync will also use that.