Bound shutdown hooks by their stop context (closes #102)

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.

internal/database/retention_lifecycle_test.go

@@ -26,6 +26,13 @@ const (
 	// reaperTestRetentionDays is the retention policy the lifecycle
 	// tests give their webhook.
 	reaperTestRetentionDays = 30
+
+	// reaperWedgeStopTimeout is the stop timeout the wedged-shutdown
+	// test hands OnStop, standing in for fx's StopTimeout. The test
+	// asserts only that the hook returns at all, and allows it
+	// reaperStopTimeout — forty times this budget — to do so, so no
+	// assertion races the wall clock.
+	reaperWedgeStopTimeout = 250 * time.Millisecond
 )
 
 // recordingLifecycle is a minimal fx.Lifecycle that records the
@@ -207,3 +214,59 @@ func TestRetentionReaper_StopHookStopsLoop(t *testing.T) {
 		"a stopped reaper must not sweep anything",
 	)
 }
+
+// TestRetentionReaper_StopHookHonoursStopTimeout is the
+// regression test for a shutdown that could never complete. fx
+// hands OnStop a context carrying the application's stop timeout;
+// an OnStop that discards it and calls wg.Wait() bare hangs the
+// process forever on a sweep blocked on a locked SQLite database
+// — precisely when a bounded shutdown matters most.
+//
+// The wedged goroutine here never observes cancellation, so the
+// hook can only return by honouring its context, and it must say
+// so rather than reporting a clean stop.
+func TestRetentionReaper_StopHookHonoursStopTimeout(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	env := setupRetentionTest(t)
+
+	env.reaper.ExportSetInterval(reaperTestInterval)
+
+	lc := startReaperViaHook(t, env.reaper)
+
+	release := make(chan struct{})
+
+	t.Cleanup(func() { close(release) })
+
+	env.reaper.ExportWedgeLoop(release)
+
+	stopCtx, cancel := context.WithTimeout(
+		context.Background(), reaperWedgeStopTimeout,
+	)
+	defer cancel()
+
+	var stopErr error
+
+	stopped := make(chan struct{})
+
+	go func() {
+		defer close(stopped)
+
+		stopErr = lc.hooks[0].OnStop(stopCtx)
+	}()
+
+	select {
+	case <-stopped:
+	case <-time.After(reaperStopTimeout):
+		t.Fatal(
+			"OnStop did not return: it discarded the stop " +
+				"context and is waiting on a wedged goroutine " +
+				"that will never observe cancellation",
+		)
+	}
+
+	require.ErrorIs(t, stopErr, context.DeadlineExceeded)
+	require.ErrorContains(t, stopErr, "retention reaper")
+}