feat(i18n): add browser-language localization with Jinja2 _() and locale middleware

Add i18n support to the blog web UI with 4 languages (en/ru/fr/de),
80 translation keys, automatic Accept-Language detection, persistent
locale cookie, and a language switcher dropdown in the header.

- Infrastructure: TranslationService, translation dicts, convenience _()
- Presentation: locale middleware, /web/lang/{locale} switcher route
- Templates: all 9 templates use {{ _(key, current_locale) }}
- Tests: 26 tests across TranslationService, locale detection helpers
- Docs: TEST_MODEL.md and FEATURE_INFRASTRUCTURE.md updated with TC-UNIT-811-821

app/infrastructure/i18n/__init__.py

@@ -0,0 +1,9 @@
+"""Internationalization support for the blog application.
+
+This package provides translation dictionaries and the translation service
+for localizing the web UI into multiple languages.
+"""
+
+from app.infrastructure.i18n.translator import SUPPORTED_LOCALES, TranslationService, _
+
+__all__ = ["SUPPORTED_LOCALES", "TranslationService", "_"]