feat: enrich member/comment/propose APIs with usernames

- ProjectMemberResponse now includes username and full_name
- Comment list endpoint returns author_username
- ProposeResponse now includes created_by_username
- All serializers resolve User objects to surface human-readable names
- Supports frontend code-first migration (TODO §3.1/3.2)

app/api/routers/comments.py

@@ -50,9 +50,22 @@ def create_comment(comment: schemas.CommentCreate, db: Session = Depends(get_db)
     return db_comment
 
 
-@router.get("/tasks/{task_id}/comments", response_model=List[schemas.CommentResponse])
+@router.get("/tasks/{task_id}/comments")
 def list_comments(task_id: int, db: Session = Depends(get_db)):
-    return db.query(models.Comment).filter(models.Comment.task_id == task_id).all()
+    comments = db.query(models.Comment).filter(models.Comment.task_id == task_id).all()
+    result = []
+    for c in comments:
+        author = db.query(models.User).filter(models.User.id == c.author_id).first()
+        result.append({
+            "id": c.id,
+            "content": c.content,
+            "task_id": c.task_id,
+            "author_id": c.author_id,
+            "author_username": author.username if author else None,
+            "created_at": c.created_at,
+            "updated_at": c.updated_at,
+        })
+    return result
 
 
 @router.patch("/comments/{comment_id}", response_model=schemas.CommentResponse)