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/development-progress-log.md

@@ -886,7 +886,7 @@ just regen-migrations <name>
 **Checklist:**
 1. ✅ Rauthy running: `docker compose ps`
 2. ✅ Client created in Rauthy admin panel
-3. ✅ Redirect URI matches exactly: `http://localhost:4000/auth/user/rauthy/callback`
+3. ✅ Redirect URI matches exactly: `http://localhost:4000/auth/user/oidc/callback`
 4. ✅ OIDC_CLIENT_SECRET in .env
 5. ✅ App restarted after .env update