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/translator.py

@@ -0,0 +1,78 @@
+"""Translation service for i18n support.
+
+This module provides the translation service that resolves translation keys
+to localized strings using in-memory translation dictionaries. Falls back
+from requested locale through English to the raw key.
+"""
+
+from app.infrastructure.i18n.translations import TRANSLATIONS
+
+SUPPORTED_LOCALES = frozenset({"en", "ru", "fr", "de"})
+DEFAULT_LOCALE = "en"
+
+
+class TranslationService:
+    """Service for resolving translation keys to localized strings.
+
+    Provides a singleton-like interface for translating UI strings
+    across the application. Falls back through requested locale to
+    English and finally to the raw key if no translation exists.
+
+    Attributes:
+        translations: Dictionary of locale to key to string mappings.
+    """
+
+    _instance: "TranslationService | None" = None
+
+    def __init__(self) -> None:
+        """Initialize translation service with translation data."""
+        self.translations = TRANSLATIONS
+
+    @classmethod
+    def get_instance(cls) -> "TranslationService":
+        """Get or create the singleton instance.
+
+        Returns:
+            The shared TranslationService instance.
+        """
+        if cls._instance is None:
+            cls._instance = cls()
+        return cls._instance
+
+    def get_text(self, key: str, locale: str = DEFAULT_LOCALE) -> str:
+        """Get translated text for a given key and locale.
+
+        Resolves the key through the locale chain: requested locale,
+        then English fallback, then the raw key itself.
+
+        Args:
+            key: Translation key (e.g. ``nav.home``).
+            locale: Target locale code (e.g. ``en``, ``ru``, ``fr``, ``de``).
+
+        Returns:
+            Translated string if found, otherwise the English version
+            or the key itself as last resort.
+        """
+        locale_translations = self.translations.get(locale)
+        if locale_translations is not None and key in locale_translations:
+            return locale_translations[key]
+
+        if locale != DEFAULT_LOCALE:
+            fallback = self.translations.get(DEFAULT_LOCALE, {}).get(key)
+            if fallback is not None:
+                return fallback
+
+        return key
+
+
+def _(key: str, locale: str = DEFAULT_LOCALE) -> str:
+    """Convenience function for translating a single key.
+
+    Args:
+        key: Translation key to look up.
+        locale: Target locale code.
+
+    Returns:
+        Translated string or the key itself if no translation found.
+    """
+    return TranslationService.get_instance().get_text(key, locale)