fixed a small database issue with price drop tracking and added doc strings

src/monitor_utils/database.py

@@ -33,7 +33,7 @@ def get_mongo_client() -> AsyncIOMotorClient:
 async def get_product_from_database(
     collection: AsyncIOMotorCollection,
     product_id: str,
-) -> dict | None:
+) -> dict:
     return await collection.find_one({"_id": product_id})
 
 
@@ -41,7 +41,7 @@ async def add_products_to_database(
     products: list[Product],
     collection: AsyncIOMotorCollection,
     category_identifier: str,
-) -> None:
+):
     if not products:
         return