fix: reduce join code length for event creation

The docs say that each byte is roughly 1.3 characters and from some testing, it seems that it consistently outputs 4 characters

backend/podium/routers/events.py

@@ -96,7 +96,7 @@ def create_event(
 
     # Generate a unique join code by continuously generating a new one until it doesn't match any existing join codes
     while True:
-        join_code = token_urlsafe(4).upper()
+        join_code = token_urlsafe(3).upper()
         if not db.events.first(formula=match({"join_code": join_code})):
             event.join_code = join_code
             break