Add invitation code validation and auto-generation for guests

Validate uniqueness of invitation codes during guest creation to prevent duplicates. Automatically generate an 8-character code if none is provided, ensuring consistent data handling. Updated tests and schemas to support these changes.

backend/app/schemas/guests.py

@@ -21,7 +21,7 @@ class GuestBase(BaseModel):
 
 
 class GuestCreate(GuestBase):
-    invitation_code: str
+    invitation_code: Optional[str] = None
 
 
 class GuestUpdate(BaseModel):
@@ -45,4 +45,5 @@ class GuestRead(GuestBase):
     response_date: Optional[datetime] = None
     actual_additional_guests: int
     is_blocked: bool
-    model_config = ConfigDict(from_attributes=True)
+    model_config = ConfigDict(from_attributes=True)
+    invitation_code: str