fix: matches_prefered_session wrong api variable

Now it uses `start_timestamp` instead of
`start_datetime` which is no presented in the API.

book_crossfit.py

@@ -243,7 +243,7 @@ class CrossFitBooker:
     def is_session_bookable(self, session: Dict, current_time: datetime) -> bool:
         """Check if a session is bookable based on user_info, ignoring error codes."""
         user_info = session.get("user_info", {})
-        
+
         # First check if can_join is true (primary condition)
         if user_info.get("can_join", False):
             return True
@@ -273,7 +273,7 @@ class CrossFitBooker:
     def matches_preferred_session(self, session: Dict, current_time: datetime) -> bool:
         """Check if session matches one of your preferred sessions"""
         try:
-            session_time = datetime.strptime(session["start_datetime"], "%Y-%m-%d %H:%M:%S")
+            session_time = datetime.strptime(session["start_timestamp"], "%Y-%m-%d %H:%M:%S")
             session_time = pytz.timezone(TIMEZONE).localize(session_time)
 
             # Check if session is exactly 2 days from now