feat(PBI-78): cost-analysis widget on insights page (#187)

* feat(PBI-78): cost-analysis data layer (T-902)

- New lib/insights/cost-analysis.ts with 5 query functions:
  getCostKpi, getCostByDay, getCostByModel, getCostByKind, getCacheEfficiency
- Period type: 7d | 30d | 90d | mtd
- Cost formula reused from token-stats.ts (input + output + cache + thinking)
- Cache savings: cache_read_tokens × (input_price - cache_read_price) / 1M
- Plan: docs/plans/PBI-78-cost-analysis-widget.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(PBI-78): cost-analysis UI component (T-903)

- New app/(app)/insights/components/cost-analysis.tsx
- Period selector (7d/30d/90d/MTD) via URL ?period= with useTransition + router.replace
- KPI strip: total cost, avg per day, cache savings, top model
- 2x2 chart grid: daily cost (BarChart), per-model + per-kind (vertical BarCharts), cache efficiency (PieChart)
- Empty state for kpi.jobCount === 0
- Uses MD3 tokens (var(--chart-N), var(--status-done))

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(PBI-78): wire CostAnalysisCard onto insights page (T-904)

- Parse ?period= from searchParams (default 30d, validates against 7d/30d/90d/mtd)
- Parallel-fetch 5 cost queries via Promise.all alongside existing widgets
- New "Cost analyse" section between Sprint Health and Plan-quality
- Existing TokenUsageCard ("Token gebruik" section) stays as sprint detail

verify (lint+typecheck+test) and build pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/INDEX.md

@@ -54,6 +54,7 @@ Auto-generated on 2026-05-10 from front-matter and headings.
 | [M9 — Actief Product Backlog](./plans/M9-active-product-backlog.md) | active | 2026-05-03 |
 | [PBI-11 — Mobile-shell met landscape-lock (settings + backlog + solo)](./plans/PBI-11-mobile-shell.md) | — | — |
 | [PBI-75 — Sprint task-edit client-side via workspace-store](./plans/PBI-75-sprint-task-edit-store.md) | — | — |
+| [PBI-78 — Cost-analyse widget op Insights-pagina](./plans/PBI-78-cost-analysis-widget.md) | — | — |
 | [Queue-loop verplaatsen van Claude naar runner](./plans/queue-loop-extraction.md) | — | — |
 | [Advies - SprintRun, PR en worktree lifecycle als state machines](./plans/sprint-pr-worktree-state-machines.md) | draft | 2026-05-06 |
 | [ST-1109 — PBI krijgt een status (Ready / Blocked / Done)](./plans/ST-1109-pbi-status.md) | active | 2026-05-03 |