fix(history): calculate utilisation in budget cost units

[albertovincenzi]

Problem

Historical charts divided admitted item count by ceilings expressed in cost units. A message costing 7 appeared to consume 1 unit, understating utilisation by 7x. Target rollups also cannot attribute aggregate traffic to a whenOp or scopeBy budget, yet the UI drew a precise-looking percentage anyway.

Fix

• return both admitted items and admitted estimated cost from the flow query;
• calculate overview utilisation from cost while retaining item counts for volume;
• ignore operation-selective budgets when choosing a target-level comparable ceiling;
• reconstruct budget history from cost_estimated rather than admitted;
• expose whenOp in graph detail so the UI can identify non-attributable histories;
• show an explicit “not attributable” explanation for scoped/conditional budgets;
• retain an admitted-count fallback for legacy rollup rows whose cost column is zero.

Review notes for Alice

No enforcement changes here. The live counter remains authoritative. This PR only prevents mixing two currencies and refuses to manufacture a historical percentage when the schema lacks the required operation/scope dimension.

The flow response adds cost but preserves admitted, total_admitted, ceiling, and utilisation for compatibility.

Verification

• cargo test -p gate-server
• cargo clippy -p gate-server --all-targets -- -D warnings
• npm run build in ui/
• direct Node assertion: 2 admitted items at cost 7 against cap 20 renders 70%, and scoped history returns unknown