[bug] Checksum generation uses wrong metadata.db path with split libraries

[dierochade]

With split-library mode enabled, imports succeed but checksum generation fails:

[ingest-processor] Error generating book checksums: no such table: books

NewBookProcessor stores the real database path in self.metadata_db, then changes self.library_dir to the split book-storage path.
Most database operations correctly use self.metadata_db, but generate_book_checksums() reconstructs the path using:
calibre_db_path = os.path.join(self.library_dir, "metadata.db")
This opens or creates an empty metadata.db inside the split storage directory, causing the books query to fail.