some comments and fixes

app/models.py

@@ -1,16 +1,19 @@
 from typing import Optional, List
 from sqlmodel import SQLModel, Field, Relationship
 
+# Relationship model
 class AuthorBookLink(SQLModel, table=True):
     author_id: int | None = Field(default=None, foreign_key="author.id", primary_key=True)
     book_id: int | None = Field(default=None, foreign_key="book.id", primary_key=True)
 
+# Author model
 class Author(SQLModel, table=True):
     id: Optional[int] = Field(primary_key=True, index=True)
     name: str
 
     books: List["Book"] = Relationship(back_populates="authors", link_model=AuthorBookLink)
 
+# Book model
 class Book(SQLModel, table=True):
     id: Optional[int] = Field(primary_key=True, index=True)
     title: str