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/api/routes/events/guests.py

@@ -13,6 +13,14 @@ router = APIRouter()
 
 @router.post("/", response_model=GuestRead, operation_id="create_guest")
 def create_guest(guest_in: GuestCreate, db: Session = Depends(get_db)):
+    if guest_in.invitation_code:
+        existing_guest = guest_crud.get_by_invitation_code(db, guest_in.invitation_code)
+        if existing_guest:
+            raise HTTPException(
+                status_code=400, detail="Guest with this invitation code already exists"
+            )
+    else:
+        guest_in.invitation_code = str(uuid.uuid4())[:8]
     guest = guest_crud.create(db, obj_in=guest_in)
     return guest