fix: Token timezone display + Skill install/uninstall bugs

backend/python/app/api/token_usage.py

@@ -252,10 +252,11 @@ def recharge(
 
 
 def _format_cst_time(dt_value):
-    """Format datetime to CST ISO string. Input is UTC naive datetime from DB."""
+    """Format datetime to CST ISO string.
+    DB values are written by cst_now() as CST time (stored naive by MySQL).
+    No UTC conversion needed - treat naive values as CST directly.
+    """
     if dt_value is None:
         return None
-    # DB stores UTC naive datetime, convert to CST
-    utc_dt = dt_value.replace(tzinfo=timezone.utc)
-    cst_dt = utc_dt.astimezone(CST_TZ)
-    return cst_dt.isoformat()
+    # DB naive values are already CST (written by cst_now()), just attach tz
+    return dt_value.replace(tzinfo=CST_TZ).isoformat()