Chapter I of the executive report needs per-project working-day count and
avg rand per working day. Instead of modifying the shared _get_labour_costs
helper (used by other sections with different column sets), enrich the
output INSIDE _build_report_context: wrap the raw result and add
working_days (distinct work-log dates per project) and avg_per_working_day
(total_cost / working_days, null-safe).
Also attaches start_date from the Project model (may be None if not set).
1 test added.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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