feat: add comments feature with nested replies and recursive rendering

Implement full comments system: domain entities (Comment, CommentLike),
value objects (CommentContent), use cases (CRUD, like toggle), SQLAlchemy
repository, API v1 endpoints, web UI with comment form and nested replies,
i18n translations (EN/RU/FR/DE), and E2E tests.

Fix nested reply (reply-to-reply) not displaying — the flat reply_comments
dict was only queried for top-level comment IDs, so deeply nested replies
were saved to DB (incrementing comment count) but never rendered. Switch
to a recursive Jinja2 macro that renders any nesting depth.

app/infrastructure/i18n/translations.py

@@ -43,6 +43,16 @@ TRANSLATIONS: dict[str, dict[str, str]] = {
         "post.edit": "Edit",
         "post.delete": "Delete",
         "post.delete_confirm": "Are you sure you want to delete this post?",
+        "post.comments": "Comments",
+        "post.write_comment": "Write a Comment",
+        "post.comment_placeholder": "Write a comment... Markdown is supported.",
+        "post.replying_to": "Replying to",
+        "post.cancel_reply": "Cancel reply",
+        "post.cancel": "Cancel",
+        "post.submit_comment": "Submit Comment",
+        "post.reply": "Reply",
+        "post.no_comments": "No comments yet. Be the first to comment!",
+        "post.comment_error": "Failed to post comment. Please try again.",
         "post_form.title_edit": "Edit Post",
         "post_form.title_new": "New Post",
         "post_form.page_title_edit": "Edit Post",
@@ -125,6 +135,16 @@ TRANSLATIONS: dict[str, dict[str, str]] = {
         "post.edit": "Редактировать",
         "post.delete": "Удалить",
         "post.delete_confirm": "Вы уверены, что хотите удалить эту статью?",
+        "post.comments": "Комментарии",
+        "post.write_comment": "Написать комментарий",
+        "post.comment_placeholder": "Напишите комментарий... Поддерживается Markdown.",
+        "post.replying_to": "Ответ",
+        "post.cancel_reply": "Отменить ответ",
+        "post.cancel": "Отмена",
+        "post.submit_comment": "Отправить",
+        "post.reply": "Ответить",
+        "post.no_comments": "Пока нет комментариев. Будьте первым!",
+        "post.comment_error": "Не удалось отправить комментарий. Попробуйте снова.",
         "post_form.title_edit": "Редактировать статью",
         "post_form.title_new": "Новая статья",
         "post_form.page_title_edit": "Редактировать статью",
@@ -207,6 +227,16 @@ TRANSLATIONS: dict[str, dict[str, str]] = {
         "post.edit": "Modifier",
         "post.delete": "Supprimer",
         "post.delete_confirm": "Êtes-vous sûr de vouloir supprimer cet article ?",
+        "post.comments": "Commentaires",
+        "post.write_comment": "Écrire un commentaire",
+        "post.comment_placeholder": "Écrivez un commentaire... Markdown est supporté.",
+        "post.replying_to": "Répondre à",
+        "post.cancel_reply": "Annuler la réponse",
+        "post.cancel": "Annuler",
+        "post.submit_comment": "Soumettre",
+        "post.reply": "Répondre",
+        "post.no_comments": "Aucun commentaire pour le moment. Soyez le premier !",
+        "post.comment_error": "Échec de l'envoi du commentaire. Veuillez réessayer.",
         "post_form.title_edit": "Modifier l'article",
         "post_form.title_new": "Nouvel article",
         "post_form.page_title_edit": "Modifier l'article",
@@ -289,6 +319,16 @@ TRANSLATIONS: dict[str, dict[str, str]] = {
         "post.edit": "Bearbeiten",
         "post.delete": "Löschen",
         "post.delete_confirm": "Sind Sie sicher, dass Sie diesen Beitrag löschen möchten?",
+        "post.comments": "Kommentare",
+        "post.write_comment": "Kommentar schreiben",
+        "post.comment_placeholder": "Schreiben Sie einen Kommentar... Markdown wird unterstützt.",
+        "post.replying_to": "Antwort an",
+        "post.cancel_reply": "Antwort abbrechen",
+        "post.cancel": "Abbrechen",
+        "post.submit_comment": "Absenden",
+        "post.reply": "Antworten",
+        "post.no_comments": "Noch keine Kommentare. Seien Sie der Erste!",
+        "post.comment_error": "Kommentar konnte nicht gesendet werden. Bitte versuchen Sie es erneut.",
         "post_form.title_edit": "Beitrag bearbeiten",
         "post_form.title_new": "Neuer Beitrag",
         "post_form.page_title_edit": "Beitrag bearbeiten",