Investigate canceled savings breakdown reads on v0.155.1-beta.1 #1831

On v0.155.1-beta.1 (Core commit 16c4ad090d3ad7a4735f473d4e0991b97593dd43), one new handleSavingsDaily: DailyCostBreakdown failed warning reported context canceled. Its journal timestamp is 25 October 2026 at 07:57:29.317 UTC.

In that exact source, go/internal/api/api_savings.go:209 and :224 use the same log text in two branches. Both pass the HTTP request context to DailyCostBreakdownContext, log the error and attempt an HTTP 500 response. The observed log does not identify the branch, who canceled the request, the inner database step or the response the client received.

First distinguish an ordinary client disconnect from a read that prevented the client from obtaining savings. Add tests for cancellation and a successful later read before changing behavior or log severity. No numeric SQLite code, shared cause with #1826/#1554, missing stored data or durable recovery was established. Writer and maintenance failure counters did not increase during the observed period. Keep private site data and raw request/database values out of the report.

This is a separate investigation handoff, not a confirmed persistence defect or a tested fix.