feat: add guard clause to improve signup process

backend/apps/web/routers/auths.py

@@ -91,9 +91,15 @@ async def signin(form_data: SigninForm):
 
 @router.post("/signup", response_model=SigninResponse)
 async def signup(request: Request, form_data: SignupForm):
-    if request.app.state.ENABLE_SIGNUP:
-        if validate_email_format(form_data.email.lower()):
-            if not Users.get_user_by_email(form_data.email.lower()):
+    if not request.app.state.ENABLE_SIGNUP:
+        raise HTTPException(400, detail=ERROR_MESSAGES.ACCESS_PROHIBITED)
+        
+    if not validate_email_format(form_data.email.lower()):
+        raise HTTPException(400, detail=ERROR_MESSAGES.INVALID_EMAIL_FORMAT)
+        
+    if Users.get_user_by_email(form_data.email.lower()):
+        raise HTTPException(400, detail=ERROR_MESSAGES.EMAIL_TAKEN)
+        
     try:
         role = "admin" if Users.get_num_users() == 0 else "pending"
         hashed = get_password_hash(form_data.password)
@@ -119,14 +125,6 @@ async def signup(request: Request, form_data: SignupForm):
     except Exception as err:
         raise HTTPException(500,
                             detail=ERROR_MESSAGES.DEFAULT(err))
-            else:
-                raise HTTPException(400, detail=ERROR_MESSAGES.EMAIL_TAKEN)
-        else:
-            raise HTTPException(400,
-                                detail=ERROR_MESSAGES.INVALID_EMAIL_FORMAT)
-    else:
-        raise HTTPException(400, detail=ERROR_MESSAGES.ACCESS_PROHIBITED)
-
 
 ############################
 # ToggleSignUp