Adds a design doc for capturing weekly plant-equipment hours via an Excel
template against the existing /dashboard/plant page. One new
plant_hours_log collection, one vehicles.unit_type field, three new
routes (import, template download, exception report CSV), and an
extension to PlantView.
Spec covers: lenient import with per-row errors, downloadable exception
report for fix-and-retry, snapshot of vehicle.unit_type on each row,
over-budget flag against a per-week budget, and a symmetric attention
panel on /dashboard/plant for type mismatches.
cc0abfdc2c