Fix initialization check in ModelManager to use class variable for singleton pattern

app/services/poetry_generation.py

@@ -12,7 +12,7 @@ class ModelManager:
         return cls._instance
     
     def __init__(self):
-        if not self._initialized:
+        if not ModelManager._initialized:
             model_name = "meta-llama/Llama-3.2-3B-Instruct"
             
             # Initialize tokenizer and model