Remove unused imports and clean up code structure across multiple files

src/backend/thread_bookgrabber.py

@@ -8,7 +8,9 @@ from loguru import logger as log
 
 logger = log
 logger.remove()
-logger.add("logs/bookgrabber_thread.log", rotation="1 week", enqueue=True)
+logger.add(
+    "logs/bookgrabber_thread.log", rotation="1 week", retention="1 month", enqueue=True
+)
 log.add(
     "logs/application.log",
     rotation="1 day",