Computes company-wide avg daily and monthly labour cost for the
executive report's hero KPI band (cards 3 and 4). Denominator is
working days (distinct work-log dates), not calendar days — true
cost-of-a-productive-day metric per design section 2.
Monthly = daily * 30.44 (the 365.25/12 month-length approximation,
which keeps annualised totals correct on average).
Tests cover: empty DB returns zero, known values with assertAlmostEqual
for the 30.44 multiplication, and that multiple workers on one date
count as 1 working day (not N).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
e74f48f050