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/templates/partials/footer.html

@@ -1,14 +1,14 @@
 <footer class="site-footer" data-testid="site-footer">
     <div class="container" data-testid="footer-container">
         <div class="footer-copyright" data-testid="footer-copyright">
-            <span data-testid="copyright-text">© 2026 Blog. All rights reserved.</span>
+            <span data-testid="copyright-text">{{ _('footer.copyright', current_locale) }}</span>
         </div>
         
         <nav class="footer-links" data-testid="footer-nav" aria-label="Footer navigation">
-            <a href="/about" class="footer-link" data-testid="footer-link-about">About</a>
-            <a href="/privacy" class="footer-link" data-testid="footer-link-privacy">Privacy</a>
-            <a href="/terms" class="footer-link" data-testid="footer-link-terms">Terms</a>
-            <a href="/api/docs" class="footer-link" data-testid="footer-link-api">API</a>
+            <a href="/about" class="footer-link" data-testid="footer-link-about">{{ _('footer.about', current_locale) }}</a>
+            <a href="/privacy" class="footer-link" data-testid="footer-link-privacy">{{ _('footer.privacy', current_locale) }}</a>
+            <a href="/terms" class="footer-link" data-testid="footer-link-terms">{{ _('footer.terms', current_locale) }}</a>
+            <a href="/api/docs" class="footer-link" data-testid="footer-link-api">{{ _('footer.api', current_locale) }}</a>
         </nav>
     </div>
 </footer>