Handle optional book.path to support books without files

Books may not have a path (e.g., physical books, metadata-only entries).
Updated path-dependent operations to handle None gracefully:

- get_file: raise ValueError if book has no path
- update_book: skip path relocation if no path exists
- remove_files: skip filesystem cleanup if no path exists

Also fixed _save_book_files return type and removed unused imports.

backend/src/chitai/services/book.py

@@ -38,18 +38,14 @@ from chitai.database.models import (
     BookSeries,
     FileMetadata,
     Identifier,
-    BookList,
     Library,
 )
-from chitai.database.models.book_progress import BookProgress
 from chitai.schemas.book import BooksCreateFromFiles
 from chitai.services.filesystem_library import BookPathGenerator
 from chitai.services.metadata_extractor import Extractor as MetadataExtractor
 from chitai.services.utils import (
     cleanup_empty_parent_directories,
-    delete_directory,
     delete_file,
-    is_empty,
     move_dir_contents,
     move_file,
     save_image,
@@ -262,6 +258,9 @@ class BookService(SQLAlchemyAsyncRepositoryService[Book]):
             ValueError: If the file is missing or not found for the given book.
         """
         book = await self.get(book_id)
+        if book.path is None:
+            raise ValueError("Cannot download file: book has no path")
+
         for file in book.files:
             if file.id != file_id:
                 continue
@@ -342,14 +341,15 @@ class BookService(SQLAlchemyAsyncRepositoryService[Book]):
             await self._save_cover_image(data)
 
         # TODO: extract out into its own function _update_book_path
-        # Check if file path must be updated
-        path_gen = BookPathGenerator(library.root_path)
-        updated_path = path_gen.generate_path(book.to_dict() | data)
-        if str(updated_path) != book.path:
-            # TODO: Move only the files associated with the book instead of the whole directory
-            await move_dir_contents(book.path, updated_path)
-            data["path"] = str(updated_path)
-            cleanup_empty_parent_directories(Path(book.path), Path(library.root_path))
+        # Check if file path must be updated (only for books with files)
+        if book.path is not None:
+            path_gen = BookPathGenerator(library.root_path)
+            updated_path = path_gen.generate_path(book.to_dict() | data)
+            if str(updated_path) != book.path:
+                # TODO: Move only the files associated with the book instead of the whole directory
+                await move_dir_contents(book.path, updated_path)
+                data["path"] = str(updated_path)
+                cleanup_empty_parent_directories(Path(book.path), Path(library.root_path))
 
         return await super().update(data, item_id=book_id, execution_options={"populate_existing": True})
 
@@ -367,8 +367,8 @@ class BookService(SQLAlchemyAsyncRepositoryService[Book]):
         book = await self.get(book_id)
         data = book.to_dict()
         data["files"] = files
-        await self._save_book_files(library, data)
-        book.files.extend(data["files"])
+        new_files = await self._save_book_files(library, data)
+        book.files.extend(new_files)
         await self.update_book(book.id, {"files": [file for file in book.files]}, library)
 
     async def remove_files(
@@ -387,7 +387,7 @@ class BookService(SQLAlchemyAsyncRepositoryService[Book]):
         """
         book = await self.get_one(Book.id == book_id, Book.library_id == library.id)
 
-        if delete_files:
+        if delete_files and book.path is not None:
             # TODO: Extract this out into its own function
             for file in (file for file in book.files if file.id in file_ids):
                 full_path = Path(book.path) / Path(file.path)
@@ -446,7 +446,7 @@ class BookService(SQLAlchemyAsyncRepositoryService[Book]):
     
         return data
 
-    async def _populate_with_unique_relationships(self, data: ModelDictT[Book]):
+    async def _populate_with_unique_relationships(self, data: ModelDictT[Book]) -> ModelDictT[Book]:
         """
         Ensure relationship entities (authors, series, tags, etc.) are unique in the database.
 
@@ -508,7 +508,7 @@ class BookService(SQLAlchemyAsyncRepositoryService[Book]):
 
         return model_data
 
-    async def _save_book_files(self, library: Library, data: dict) -> dict:
+    async def _save_book_files(self, library: Library, data: dict) -> list[FileMetadata]:
         """
         Save uploaded book files to the filesystem.
 
@@ -558,7 +558,7 @@ class BookService(SQLAlchemyAsyncRepositoryService[Book]):
             )
 
         data["files"] = file_metadata
-        return data
+        return data["files"]
 
     async def _parse_metadata_from_files(self, data: dict, root_path: Path | None = None) -> dict:
         """