Patch dashboard runs from SSE payloads instead of full refetch (BEN-49)

[benSepanski]

Context

BEN-49: src/web/Dashboard.tsx refetched /api/runs and /api/events/recent on every SSE event except usage/tick/settings, so a live run with frequent turn events caused per-event table rebuilds, flicker, and wasted bandwidth.

TL;DR

Patch the dashboard runs list in-memory from the SSE event payload; only fall back to /api/runs for unknown runIds, and use /api/runs/:id to top up token totals when a run finishes.

Summary

• New src/web/dashboardEvents.ts — pure helpers applyTurnEvent, applyRunFinishedEvent, replaceRun, hasRun. No React deps. 11 unit tests in dashboardEvents.test.ts.
• src/web/Dashboard.tsx::useEventStream — turn increments turnCount on the matching row in-memory; only falls back to GET /api/runs if the runId is unknown (stale tab).
• runFinished stamps status + finishedAt immediately for instant feedback, then a single GET /api/runs/:id fills in authoritative tokens + cost via replaceRun. Recent-events feed is refreshed once per finish (so new error rows surface).
• runStarted keeps the existing GET /api/runs fallback (rare event; SSE payload is Issue-shaped, not ApiRun-shaped).
• docs/FRONTEND.md documents the new SSE → state convention next to the data-sources table.

Demo

n/a — automated cron run; no browser MCP in this environment. Manual smoke-test: pnpm dev WORKFLOW.md --mock, open the dashboard during a long mock run, verify devtools Network shows no GET /api/runs per turn SSE event (only on runFinished → one /api/runs/:id plus /api/events/recent), and the runs <tbody> no longer flickers / scroll-jumps as turns stream in.

Alternatives

• Debounce the existing full refetch to ~300ms per burst — rejected: the ticket calls out incremental updates as the preferred fix and the payload already carries the runId we need.
• Targeted /api/runs/:id on every turn — rejected: turn events fire many times per second per agent; we'd just be moving the load from /api/runs to /api/runs/:id. Pure in-memory turnCount++ is enough; authoritative tokens land at runFinished.
• Reactive store (Zustand / Jotai) — rejected: out of scope, and a single useState<ApiRun[]> plus pure reducer matches the dashboard's existing patterns.

Test Plan

• pnpm all — typecheck + fmt:check + lint + test + eval (131 unit + 5 eval, all green)
• pnpm build:web — bundle builds (184ms)
• New dashboardEvents.test.ts covers turn-increment, no-op when runId is missing, finished-status update, finishedAt-preservation when already set, replaceRun by id, and hasRun lookups.

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