Headcount at every month end

For each month of 2024, count how many people were employed on the last day of that month.

Someone counts on a given day if they were hired on or before that day, and either haven't left or left after that day. Leaving on the last day of the month means they're not counted at that month end.

Return 12 rows, month_end as a date (2024-01-31, 2024-02-29, ...), even for months where the headcount is 0.