feat(pricing): model-aware cost calculation for dashboard

[spboyer]

Closes #308

Summary

Replaces the flat $0.00025/token dashboard cost estimate with a three-tier pricing pipeline:

1. sdk — use the per-request cost the Copilot SDK already reports (ModelUsage.RequestCost), when available.
2. table — price input / output / cache_read / cache_write tokens against a per-model rate table (Claude 3.5/4.x, GPT‑5 family, GPT‑4o/4.1, Gemini 2.5).
3. estimate — fall back to the original flat $0.00025/token rate when the model is unknown.

The chosen source is propagated as costSource through RunSummary / SummaryResponse so the dashboard can show an info tooltip explaining how the number was computed.

What changed

Backend

• New internal/pricing package — Rates struct, LookupRates (longest-prefix match with a word-boundary check so gpt-4 doesn't match gpt-40-future), Compute(*UsageStats) → (dollars, source), CombineSources([]string), and a model rate table with effective date 2025-01-01.
• internal/webapi/store.go, storage_adapter.go — replaced estimateCost with pricing.Compute(models.AggregateUsageStats(...)). Summary() now collects per-run sources and combines them with CombineSources.
• internal/webapi/types.go — added CostSource string to RunSummary and SummaryResponse.
• Tests — comprehensive internal/pricing unit tests (SDK path, table path with cache tokens, multi-model aggregation, unknown-model fallback, prefix-boundary safety, CombineSources). New handlers_test cases assert costSource round-trips through the HTTP layer and aggregates correctly when mixed.

Frontend

• web/src/components/InfoTooltip.tsx — small lucide-react Info icon with a native title tooltip.
• web/src/lib/format.ts — costSourceTooltip(source?) helper centralizes the four wordings (sdk, table, estimate, mixed).
• web/src/api/client.ts — added optional costSource?: CostSource to RunSummary and SummaryResponse.
• RunsTable — info icon in the Cost column header; per-row title reveals the source per run.
• KPICards — info icon next to "Avg Cost" reflecting the aggregate source.
• RunDetail — info icon next to the Cost stat card.

Docs

• site/src/content/docs/guides/dashboard.mdx — new "Cost accuracy" section explaining the three sources, when each applies, the rate-table effective date, and a note that the table is best-effort.

Validation

• go test ./... — all packages pass (internal/pricing, internal/webapi including new costSource tests).
• go vet ./... — clean. make lint reports golangci-lint not installed, skipping... on this host; CI will run it.
• cd web && npx tsc -b && npm run build — clean.
• cd web && npx playwright test --project=chromium — 52/52 passed (screenshots regenerated to include the new info icons).

Design notes / follow-ups

• The rate table is intentionally embedded as a Go map for simplicity. The values are best-effort list pricing snapshots; a sensible follow-up would be externalizing them to YAML/JSON with a published effective-date metadata file (or pulling from a provider catalog). I left a note in the dashboard guide.
• LiveView, CompareView, and TrendsPage show per-run costs that already benefit from the new pricing pipeline (their values come from the same RunSummary). I scoped the tooltip wiring to the three primary surfaces called out in the issue to keep the diff focused; happy to extend if reviewers prefer.
• resultSummaryToRunSummary in storage_adapter.go still hardcodes Cost: 0 for the ResultSummary legacy path — that struct doesn't carry token data, so there's nothing to price. Left unchanged; CostSource is omitted (omitempty) for those rows.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

This PR replaces the dashboard’s flat per-token cost estimate with a model-aware pricing pipeline (SDK-reported cost → per-model rate table → flat fallback), and surfaces the chosen costSource end-to-end so the UI can disclose how the cost was derived.

Changes:

• Added internal/pricing with rate-table lookup + cost computation + source aggregation, and integrated it into internal/webapi summaries.
• Plumbed costSource through API types/tests and into the web UI (Runs table, KPI cards, run detail) via tooltip affordances.
• Documented cost accuracy behavior in the dashboard guide and updated embedded web build output.

Show a summary per file

File	Description
web/src/lib/format.ts	Adds costSourceTooltip() used by UI tooltips.
web/src/components/RunsTable.tsx	Adds cost header info icon + per-row cost source hover tooltip.
web/src/components/RunDetail.tsx	Adds cost source info icon in the run detail stat card; extends StatCard.
web/src/components/KPICards.tsx	Adds cost source info icon to “Avg Cost”; extends Card.
web/src/components/InfoTooltip.tsx	Introduces a reusable inline info icon component.
web/src/api/client.ts	Adds CostSource union + optional costSource fields in API types.
web/dist/index.html	Updates embedded build asset references.
site/src/content/docs/guides/dashboard.mdx	Documents “Cost accuracy” sources and caveats.
internal/webapi/types.go	Adds CostSource fields to API responses with docs.
internal/webapi/store.go	Replaces estimateCost() with pricing.Compute() using aggregated usage.
internal/webapi/storage_adapter.go	Aggregates/returns CostSource for summary responses.
internal/webapi/handlers_test.go	Adds/updates tests to assert costSource behavior/round-trip.
internal/pricing/pricing.go	New pricing engine: rate table + lookup + compute + CombineSources.
internal/pricing/pricing_test.go	Unit tests covering SDK/table/estimate paths + prefix-boundary + combining sources.

Copilot's findings

• Files reviewed: 13/18 changed files
• Comments generated: 4