Measure ACT worker and push behavior under load

Problem/Motivation

We do not yet have measured operating limits or tuning guidance for the new background and push paths.

Proposed resolution

Measure small and large audiences with representative thread sizes and write rates. Compare demand rebuilding, background warming and push delivery.

Remaining tasks

Record rebuild counts, request bursts, worker memory and update latency. Test long-running workers and alternative backends, then document practical limits and recommended settings.