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.

tests/TEST_MODEL.md

@@ -23,6 +23,7 @@ adding new tests.
 | Post Delete via Web | — | — | — | 40% | P1 | ⚠️ Partial |
 | i18n Localization | 100% | — | — | — | P1 | ✅ Active |
 | Post Likes | 100% | — | 100% | — | P1 | ✅ Active |
+| Comments (CRUD, Like, Nested) | — | — | — | — | P1 | 🔴 In Progress |
 
 Legend: ✅ Covered / ⚠️ Partial / ❌ Missing / — Not Applicable
 
@@ -36,6 +37,7 @@ Legend: ✅ Covered / ⚠️ Partial / ❌ Missing / — Not Applicable
 | Infrastructure & Bootstrap | [FEATURE_INFRASTRUCTURE.md](FEATURE_INFRASTRUCTURE.md) |
 | i18n Localization | [FEATURE_INFRASTRUCTURE.md](FEATURE_INFRASTRUCTURE.md) |
 | Post Likes | [FEATURE_LIKES.md](FEATURE_LIKES.md) |
+| Comments | [FEATURE_COMMENTS.md](FEATURE_COMMENTS.md) |
 
 ## Test Naming Convention