docs(issue-225): add backend i18n baseline artifact

docs/ISSUE-225-BACKEND-I18N-BASELINE-ARTIFACT.md

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+# ISSUE-225 Artifact — Backend i18n baseline (resolver + fallback)
+
+Issue: **#225** (`[MVP][READY] #175-A: Backend i18n baseline (resolver + fallback)`)
+
+## Scope verified
+
+- Backend locale resolver supports `da` + `en` and normalizes language tags.
+- Unsupported locale requests fall back to default locale (`en`).
+- Error payload contract is stable for API errors:
+  - `error_code`
+  - `error`
+  - `locale`
+
+## Implementation references
+
+- Locale resolution + fallback chain:
+  - `lobby/i18n.py`
+  - `resolve_locale()`
+  - `resolve_error_message()`
+  - `api_error()`
+- Shared locale contract source:
+  - `shared/i18n/lobby.json` (`locales.default=en`, supported includes `en`, `da`)
+
+## Acceptance checks run
+
+Command:
+
+```bash
+.venv/bin/python manage.py test \
+  lobby.tests.I18nResolverTests \
+  lobby.tests.LobbyFlowTests \
+  lobby.tests.StartRoundTests
+```
+
+Result:
+
+- `Ran 28 tests ... OK`
+- Confirms resolver behavior for locale normalization + fallback and stable error payload fields across flow endpoints.
+
+## Notes
+
+- Existing merged follow-ups in `main` include Accept-Language parsing fixes for q-values and locale tag normalization.
+- This artifact documents the final baseline state and verification evidence for #225.