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/main.py

@@ -28,6 +28,7 @@ from app.presentation.web import auth_router
 from app.presentation.web import router as web_router
 from app.presentation.web.error_handlers import register_error_handlers
 from app.presentation.web.flash import setup_flash_manager
+from app.presentation.web.locale import setup_locale_manager
 
 
 @asynccontextmanager
@@ -86,6 +87,15 @@ def app_factory() -> FastAPI:
             request.state.flash_manager.set_cookie(response)
         return response
 
+    @app.middleware("http")
+    async def locale_middleware(
+        request: Request, call_next: Callable[[Request], Awaitable[Response]]
+    ) -> Response:
+        """Middleware to detect and set locale for each request."""
+        await setup_locale_manager(request)
+        response = await call_next(request)
+        return response
+
     app.add_middleware(
         CORSMiddleware,
         allow_origins=["*"],