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/admin-bootstrap-and-oidc-role-sync.md

@@ -39,8 +39,8 @@
 
 ### Where It Runs
 
-1. Registration: register_with_rauthy after_action calls OidcRoleSync.
-2. Sign-in: sign_in_with_rauthy prepare after_action calls OidcRoleSync for each user.
+1. Registration: register_with_oidc after_action calls OidcRoleSync.
+2. Sign-in: sign_in_with_oidc prepare after_action calls OidcRoleSync for each user.
 
 ### Internal Action