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.

docs/oidc-account-linking.md

@@ -10,10 +10,10 @@ This feature implements secure account linking between password-based accounts a
 
 #### 1. Security Fix: `lib/accounts/user.ex`
 
-**Change**: The `sign_in_with_rauthy` action now filters by `oidc_id` instead of `email`.
+**Change**: The `sign_in_with_oidc` action now filters by `oidc_id` instead of `email`.
 
 ```elixir
-read :sign_in_with_rauthy do
+read :sign_in_with_oidc do
   argument :user_info, :map, allow_nil?: false
   argument :oauth_tokens, :map, allow_nil?: false
   prepare AshAuthentication.Strategy.OAuth2.SignInPreparation