Global refactoring of the project to use poetry and implement tests,

fixing bugs, changing the handling of dto and db models, preparing to
add new functionality

library_service/models/db/book.py

@@ -0,0 +1,14 @@
+from typing import List, Optional, TYPE_CHECKING
+from sqlmodel import SQLModel, Field, Relationship
+from ..dto.book import BookBase
+from .links import AuthorBookLink
+
+if TYPE_CHECKING:
+    from .author import Author
+
+class Book(BookBase, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True, index=True)
+    authors: List["Author"] = Relationship(
+        back_populates="books",
+        link_model=AuthorBookLink
+    )