fix(scouts): batch emissions + report lookups in the scout signals panel

[andrewm4894]

Problem

Opening the Signals section in a scout's detail view was slow. ScoutSignalsSection rendered one RunEmissions per visible run, and each one fired two per-run requests — runs/<id>/emissions/ and runs/<id>/emissions/reports/. With N visible emitted runs that's 2×N requests and N ClickHouse scans on open (each emissions/reports request runs its own unmaterialized JSONExtract scan over document_embeddings).

This is the desktop twin of the web fix in PostHog/posthog#66823, which replaced the same fan-out in the web app's findingsLogic.

Changes

Call the two batched endpoints added by #66823 once for the whole visible-run window:

• runs/emissions/batch — every visible run's emitted findings in one Postgres query, each row keeps its run_id.
• runs/emissions/reports/batch — resolves every run's findings to their inbox report in a single ClickHouse round-trip.

Specifically:

• api-client: new scoutPost helper; replaced listScoutRunEmissions / listScoutEmissionReports with batchScoutRunEmissions / batchScoutEmissionReports (POST a run_ids list, short-circuit on empty, handle array or paginated body).
• hooks: useScoutRunEmissions / useScoutEmissionReports now take a run-id list, key the query on the sorted ids, and use keepPreviousData so "Show more" doesn't blank the already-rendered cards while the widened window refetches.
• ScoutSignalsSection: fetches once at the section level, groups emissions by run_id locally; RunEmissions is now presentational (no hooks).

Net effect: 2×N requests / N CH scans → 2 requests / 1 CH scan on panel open.

The desktop window is small (INITIAL_EMITTED_RUNS = 10, "Show more" expands to the full emitted-runs window), well under the backend's 200-id batch cap, so no client-side chunking is needed.

⚠️ Blocked on backend

The runs/emissions/batch and runs/emissions/reports/batch endpoints are added by PostHog/posthog#66823, which is not yet merged. This PR must not ship until #66823 is merged and deployed to US + EU — otherwise the Signals panel will 404 on open. Keeping this as a draft until then.

How tested

Agent (Claude Code), human-directed. Automated only — no manual UI testing.

• pnpm --filter @posthog/api-client --filter @posthog/ui typecheck clean.
• Added 4 api-client tests (batched scout emissions): empty short-circuit, request shape + run_ids body, paginated unwrap, non-OK error. Full posthog-client.test.ts passes (34 tests).
• biome clean on all touched files.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit aaab3c6.}

[chatgpt-codex-connector]

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Reviewed commit: 4d0bb67d35

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[greptile-apps]

Comments Outside Diff (1)

1. packages/ui/src/features/scouts/components/ScoutSignalsSection.tsx, line 99-108 (link)

   Newly-revealed runs show "no signals" while fetching

   When "Show more" widens visibleRuns, keepPreviousData keeps isLoading=false and surfaces the previous (narrower) batch as data. Any run that wasn't in that previous batch returns undefined from emissionsByRunId.get(run.run_id), so RunEmissions reaches the !emissions || emissions.length === 0 branch and displays "No signal details available for this run." — but the run has emitted_count > 0, meaning the user sees incorrect content until the wider batch resolves.

   The fix is to also expose isFetching from the hook and compute a per-run loading flag: emissionsLoading || (emissionsFetching && !emissionsByRunId.has(run.run_id)). That keeps existing cards visible while giving new runs a skeleton instead of a false empty state.

_{Reviews (1): Last reviewed commit: "fix(scouts): batch emissions + report lo..." | Re-trigger Greptile}

[stamphog]

The change is behaviorally sound and well-tested, but it depends on new batch backend endpoints that can't be verified from the client-side code, and it has no human reviewer sign-off despite being a complex API contract change affecting production UI.