Process ACT background work through configurable backends

Problem/Motivation

Sites need to prepare discussions outside web requests using infrastructure they already operate.

Proposed resolution

Provide an optional Background submodule with transactional work capture, cross-request coalescing, backend plugins and bounded retries. Support cron and continuous processing through vendor/bin/dr.

Remaining tasks

Review the alpha implementation. Verify alternative queue backends, duplicate delivery, worker shutdown and recovery after interruption.