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/presentation/web/locale.py

@@ -0,0 +1,72 @@
+"""Locale detection and management for i18n support.
+
+This module provides locale detection from Accept-Language headers and cookies,
+following the same middleware pattern as the flash message system.
+"""
+
+from fastapi import Request
+
+from app.infrastructure.i18n.translator import DEFAULT_LOCALE, SUPPORTED_LOCALES
+
+LOCALE_COOKIE_NAME = "locale"
+SUPPORTED_LOCALES_SET: frozenset[str] = SUPPORTED_LOCALES
+
+
+def _parse_accept_language(header: str) -> list[str]:
+    """Parse Accept-Language header into ordered list of locale codes.
+
+    Args:
+        header: Raw Accept-Language header value.
+
+    Returns:
+        List of locale codes in preference order, with region subtags removed.
+    """
+    if not header:
+        return []
+
+    locales: list[str] = []
+    for part in header.split(","):
+        part = part.strip()
+        if not part:
+            continue
+        locale = part.split(";")[0].strip().split("-")[0]
+        if locale:
+            locales.append(locale)
+    return locales
+
+
+def _get_best_locale(request: Request) -> str:
+    """Detect the best locale for the current request.
+
+    Priority order: cookie → Accept-Language header → default.
+
+    Args:
+        request: FastAPI request object.
+
+    Returns:
+        Best matching locale code, defaulting to ``en``.
+    """
+    cookie_locale = request.cookies.get(LOCALE_COOKIE_NAME)
+    if cookie_locale and cookie_locale in SUPPORTED_LOCALES_SET:
+        return cookie_locale
+
+    accept_language = request.headers.get("accept-language", "")
+    for lang in _parse_accept_language(accept_language):
+        if lang in SUPPORTED_LOCALES_SET:
+            return lang
+
+    return DEFAULT_LOCALE
+
+
+async def setup_locale_manager(request: Request) -> None:
+    """Set the detected locale on request state.
+
+    Called early in the request lifecycle so that route handlers and
+    template rendering can access the current locale via
+    ``request.state.locale``.
+
+    Args:
+        request: FastAPI request object.
+    """
+    if not hasattr(request.state, "locale"):
+        request.state.locale = _get_best_locale(request)