feat: use message-id header to only process message once

backend/app/schemas/entries.py

@@ -5,17 +5,21 @@ from pydantic import BaseModel, ConfigDict
 
 class EntryBase(BaseModel):
     """Base schema for an entry."""
+
     subject: str
     body: str
+    message_id: str
 
 
 class EntryCreate(EntryBase):
     """Schema for creating a new entry."""
+
     pass
 
 
 class Entry(EntryBase):
     """Schema for retrieving an entry with its ID and newsletter ID."""
+
     id: int
     newsletter_id: int
     received_at: datetime.datetime