fx hands OnStop a context carrying the application's stop timeout,
and the delivery engine, the retention reaper, and the archive
sweeper all discarded it and called wg.Wait() bare. A worker wedged
inside a delivery target that never returns, or a sweep blocked on a
locked SQLite database, hung the process forever instead of letting
it exit when the timeout expired.
All three now wait through internal/lifecycle.WaitForShutdown, which
selects the drained WaitGroup against the stop context and, on
timeout, logs at error naming the component and returns an error
rather than reporting a clean stop.
Engine.stop also gains the cancel != nil guard its two mirrored
components already had.
The delivery engine worker pool and the retention reaper both rooted their
goroutines in the fx OnStart hook context, which fx cancels 15s into startup.
Both now use context.WithCancel(context.Background()), bounded by OnStop.
