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/crud/guest.py

@@ -11,6 +11,10 @@ import uuid
 class CRUDGuest(CRUDBase[Guest, GuestCreate, GuestUpdate]):
 
     def create(self, db, obj_in: GuestCreate):
+
+        if obj_in.invitation_code is None:
+            obj_in.invitation_code = str(uuid.uuid4())[:8]
+
         db_guest = Guest(
             event_id=uuid.UUID(obj_in.event_id) if isinstance(obj_in.event_id, str) else obj_in.event_id,  # explicit casting
             invited_by=uuid.UUID(obj_in.invited_by) if isinstance(obj_in.invited_by, str) else obj_in.invited_by,