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/author.py

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