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/pages/index.html

@@ -1,29 +1,29 @@
 {% extends "base.html" %}
 
-{% block title %}Blog - Home{% endblock %}
-{% block meta_description %}Discover stories, thinking, and expertise from writers on any topic. A modern blog built with FastAPI.{% endblock %}
-{% block meta_keywords %}blog, articles, posts, writing, fastapi, python{% endblock %}
+{% block title %}{{ _('home.title', current_locale) }}{% endblock %}
+{% block meta_description %}{{ _('home.meta_description', current_locale) }}{% endblock %}
+{% block meta_keywords %}{{ _('home.meta_keywords', current_locale) }}{% endblock %}
 
 {% block og_type %}website{% endblock %}
-{% block og_title %}Blog - Home{% endblock %}
-{% block og_description %}Discover stories, thinking, and expertise from writers on any topic.{% endblock %}
+{% block og_title %}{{ _('home.title', current_locale) }}{% endblock %}
+{% block og_description %}{{ _('home.meta_description', current_locale) }}{% endblock %}
 
-{% block twitter_title %}Blog - Home{% endblock %}
-{% block twitter_description %}Discover stories, thinking, and expertise from writers on any topic.{% endblock %}
+{% block twitter_title %}{{ _('home.title', current_locale) }}{% endblock %}
+{% block twitter_description %}{{ _('home.meta_description', current_locale) }}{% endblock %}
 
 {% block content %}
 <section class="page-header" data-testid="page-header-home">
     <div class="page-header-flex">
         <div data-testid="page-header-content">
-            <h1 class="page-title" data-testid="page-title-home">Latest Posts</h1>
-            <p class="page-subtitle" data-testid="page-subtitle-home">Discover stories, thinking, and expertise from writers on any topic.</p>
+            <h1 class="page-title" data-testid="page-title-home">{{ _('home.page_title', current_locale) }}</h1>
+            <p class="page-subtitle" data-testid="page-subtitle-home">{{ _('home.page_subtitle', current_locale) }}</p>
         </div>
         {% if can_create %}
         <a href="/web/posts/new" class="btn btn-primary" data-testid="btn-create-post-header">
             <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                 <path d="M8 2v12M2 8h12" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
             </svg>
-            Write a Post
+            {{ _('home.write_post', current_locale) }}
         </a>
         {% endif %}
     </div>
@@ -38,9 +38,9 @@
                 <a href="/web/posts/{{ post.slug }}" data-testid="post-title-link-{{ post.id }}">{{ post.title }}</a>
             </h2>
             {% if post.published %}
-            <span class="badge badge-success" data-testid="post-status-{{ post.id }}">Published</span>
+            <span class="badge badge-success" data-testid="post-status-{{ post.id }}">{{ _('home.status_published', current_locale) }}</span>
             {% else %}
-            <span class="badge" data-testid="post-status-{{ post.id }}">Draft</span>
+            <span class="badge" data-testid="post-status-{{ post.id }}">{{ _('home.status_draft', current_locale) }}</span>
             {% endif %}
         </div>
         
@@ -65,7 +65,7 @@
                 {% endfor %}
             </div>
             <a href="/web/posts/{{ post.slug }}" class="btn btn-sm" data-testid="btn-read-more-{{ post.id }}">
-                Read more
+                {{ _('home.read_more', current_locale) }}
                 <svg width="14" height="14" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-left: 0.25rem;">
                     <path d="M6 12L10 8L6 4" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
@@ -77,26 +77,26 @@
 
 <nav class="pagination" data-testid="pagination" aria-label="Pagination">
     {% if has_prev %}
-    <a href="{{ request.url.path }}?page={{ current_page - 1 }}" class="pagination-item" data-testid="pagination-prev">Previous</a>
+    <a href="{{ request.url.path }}?page={{ current_page - 1 }}" class="pagination-item" data-testid="pagination-prev">{{ _('home.pagination_previous', current_locale) }}</a>
     {% else %}
-    <span class="pagination-item disabled" data-testid="pagination-prev">Previous</span>
+    <span class="pagination-item disabled" data-testid="pagination-prev">{{ _('home.pagination_previous', current_locale) }}</span>
     {% endif %}
     
     <span class="pagination-item active" data-testid="pagination-current">{{ current_page }}</span>
     
     {% if has_next %}
-    <a href="{{ request.url.path }}?page={{ current_page + 1 }}" class="pagination-item" data-testid="pagination-next">Next</a>
+    <a href="{{ request.url.path }}?page={{ current_page + 1 }}" class="pagination-item" data-testid="pagination-next">{{ _('home.pagination_next', current_locale) }}</a>
     {% else %}
-    <span class="pagination-item disabled" data-testid="pagination-next">Next</span>
+    <span class="pagination-item disabled" data-testid="pagination-next">{{ _('home.pagination_next', current_locale) }}</span>
     {% endif %}
 </nav>
 
 {% else %}
 <div class="empty-state" data-testid="empty-state">
     <div class="empty-state-icon" data-testid="empty-state-icon">📝</div>
-    <h3 data-testid="empty-state-title">No posts yet</h3>
-    <p data-testid="empty-state-description">Be the first to write a post!</p>
-    <a href="/web/posts/new" class="btn btn-primary" data-testid="btn-create-first-post">Create your first post</a>
+    <h3 data-testid="empty-state-title">{{ _('home.empty_title', current_locale) }}</h3>
+    <p data-testid="empty-state-description">{{ _('home.empty_description', current_locale) }}</p>
+    <a href="/web/posts/new" class="btn btn-primary" data-testid="btn-create-first-post">{{ _('home.empty_action', current_locale) }}</a>
 </div>
 {% endif %}
 {% endblock %}