create QuestionType enum

* make `type` in PartialQuestions more strict

* apply QuestionType to the the `type` field of Conversation

src/codegate/api/v1_models.py

@@ -1,4 +1,5 @@
 import datetime
+from enum import Enum
 from typing import Any, List, Optional, Union
 
 import pydantic
@@ -87,6 +88,11 @@ class QuestionAnswer(pydantic.BaseModel):
     answer: Optional[ChatMessage]
 
 
+class QuestionType(str, Enum):
+    chat = "chat"
+    fim = "fim"
+
+
 class PartialQuestions(pydantic.BaseModel):
     """
     Represents all user messages obtained from a DB row.
@@ -96,7 +102,7 @@ class PartialQuestions(pydantic.BaseModel):
     timestamp: datetime.datetime
     message_id: str
     provider: Optional[str]
-    type: str
+    type: QuestionType
 
 
 class PartialQuestionAnswer(pydantic.BaseModel):
@@ -115,7 +121,7 @@ class Conversation(pydantic.BaseModel):
 
     question_answers: List[QuestionAnswer]
     provider: Optional[str]
-    type: str
+    type: QuestionType
     chat_id: str
     conversation_timestamp: datetime.datetime