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/post_form.html

@@ -1,6 +1,6 @@
 {% extends "base.html" %}
 
-{% block title %}{% if is_edit %}Edit Post{% else %}New Post{% endif %} - Blog{% endblock %}
+{% block title %}{% if is_edit %}{{ _('post_form.title_edit', current_locale) }}{% else %}{{ _('post_form.title_new', current_locale) }}{% endif %} - {{ _('base.default_title', current_locale) }}{% endblock %}
 
 {% block extra_css %}
 <link rel="stylesheet" href="/static/css/easymde.min.css" data-testid="easymde-stylesheet">
@@ -9,7 +9,7 @@
 {% block content %}
 <section class="page-header" data-testid="page-header-form">
     <h1 class="page-title" data-testid="page-title-form">
-        {% if is_edit %}Edit Post{% else %}Create New Post{% endif %}
+        {% if is_edit %}{{ _('post_form.page_title_edit', current_locale) }}{% else %}{{ _('post_form.page_title_new', current_locale) }}{% endif %}
     </h1>
 </section>
 
@@ -22,7 +22,7 @@
     <div class="card-body" data-testid="form-post-body">
         <div class="form-group" data-testid="form-group-title">
             <label for="title" class="form-label form-label-required" data-testid="label-title">
-                Title
+                {{ _('post_form.label_title', current_locale) }}
             </label>
             <input
                 type="text"
@@ -30,31 +30,31 @@
                 name="title"
                 class="input input-lg"
                 value="{% if post %}{{ post.title }}{% endif %}"
-                placeholder="Enter post title"
+                placeholder="{{ _('post_form.placeholder_title', current_locale) }}"
                 required
                 data-testid="input-title"
             >
-            <span class="form-hint" data-testid="hint-title">A catchy title for your post</span>
+            <span class="form-hint" data-testid="hint-title">{{ _('post_form.hint_title', current_locale) }}</span>
         </div>
 
         <div class="form-group" data-testid="form-group-content">
             <label for="content" class="form-label form-label-required" data-testid="label-content">
-                Content
+                {{ _('post_form.label_content', current_locale) }}
             </label>
             <textarea
                 id="content"
                 name="content"
                 rows="12"
-                placeholder="Write your post content here..."
+                placeholder="{{ _('post_form.placeholder_content', current_locale) }}"
                 required
                 data-testid="textarea-content"
             >{% if post %}{{ post.content }}{% endif %}</textarea>
-            <span class="form-hint" data-testid="hint-content">The main content of your post. Markdown is supported.</span>
+            <span class="form-hint" data-testid="hint-content">{{ _('post_form.hint_content', current_locale) }}</span>
         </div>
         
         <div class="form-group" data-testid="form-group-tags">
             <label for="tags" class="form-label" data-testid="label-tags">
-                Tags
+                {{ _('post_form.label_tags', current_locale) }}
             </label>
             <input 
                 type="text" 
@@ -62,10 +62,10 @@
                 name="tags" 
                 class="input"
                 value="{% if post %}{{ post.tags|join(', ') }}{% endif %}"
-                placeholder="python, fastapi, tutorial"
+                placeholder="{{ _('post_form.placeholder_tags', current_locale) }}"
                 data-testid="input-tags"
             >
-            <span class="form-hint" data-testid="hint-tags">Comma-separated list of tags</span>
+            <span class="form-hint" data-testid="hint-tags">{{ _('post_form.hint_tags', current_locale) }}</span>
         </div>
         
     </div>
@@ -73,15 +73,15 @@
     <div class="card-footer" data-testid="form-post-footer">
         <div class="flex justify-between items-center" data-testid="form-actions">
             <a href="{% if is_edit %}/web/posts/{{ post.slug }}{% else %}/web/{% endif %}" class="btn" data-testid="btn-cancel">
-                Cancel
+                {{ _('post_form.cancel', current_locale) }}
             </a>
             
             <div class="flex gap-2" data-testid="form-submit-actions">
                 <button type="submit" name="action" value="draft" class="btn" data-testid="btn-save-draft">
-                    Save as Draft
+                    {{ _('post_form.save_draft', current_locale) }}
                 </button>
                 <button type="submit" name="action" value="publish" class="btn btn-primary" data-testid="btn-publish-post">
-                    {% if is_edit %}Update Post{% else %}Publish Post{% endif %}
+                    {% if is_edit %}{{ _('post_form.update_post', current_locale) }}{% else %}{{ _('post_form.publish_post', current_locale) }}{% endif %}
                 </button>
             </div>
         </div>
@@ -99,7 +99,7 @@
         spellChecker: false,
         status: false,
         minHeight: '300px',
-        placeholder: 'Write your post content here...',
+        placeholder: '{{ _('post_form.placeholder_content', current_locale) }}',
         toolbar: [
             'bold', 'italic', 'heading', '|',
             'code', 'quote', 'unordered-list', 'ordered-list', '|',