feat: overhead FTE scaling — auto-compute headcount from overhead formula

[NickMonrad]

Overview

Currently, overhead resources (e.g., Project Manager at 22%) compute allocated days based on a formula (percentage of delivery effort, fixed days/week, etc.), but never translate that back into FTE headcount.

If the delivery squad grows and generates 60+ hours/week of PM overhead demand, the system still shows 1 PM — it doesn't flag that you actually need 1.5 or 2 PMs to cover the load.

Proposed Behaviour

• For each overhead-linked RT, compute: requiredFTE = overheadDays / (periodWeeks × 5)
• Show this alongside the current overhead allocation on the Resource Profile page
• Optionally auto-scale the overhead RT count when a Capacity Plan is applied
• Flag when overhead demand exceeds current headcount capacity

Context

Raised during Squad Planner design (PR #239 / issue #233). The Squad Planner only capacity-plans delivery RTs (those with actual task demand). Overhead scaling is a separate layer.

Acceptance Criteria

• Resource Profile shows computed FTE need for overhead-linked RTs
• Warning when overhead demand exceeds headcount
• Optional auto-scale on Capacity Plan apply