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/lifecycle/lifecycle_test.go

@@ -0,0 +1,62 @@
+package lifecycle_test
+
+import (
+	"context"
+	"log/slog"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/lifecycle"
+)
+
+// waitTimeout is the stop budget the timeout case gives a
+// goroutine that never returns. The test's own patience is the
+// go test deadline, so the only thing this value affects is how
+// long the case takes.
+const waitTimeout = 100 * time.Millisecond
+
+func discardLogger() *slog.Logger {
+	return slog.New(slog.DiscardHandler)
+}
+
+func TestWaitForShutdown_DrainedGroup(t *testing.T) {
+	t.Parallel()
+
+	var wg sync.WaitGroup
+
+	wg.Go(func() {})
+
+	require.NoError(
+		t,
+		lifecycle.WaitForShutdown(
+			context.Background(), discardLogger(),
+			"test component", &wg,
+		),
+	)
+}
+
+func TestWaitForShutdown_ContextExpires(t *testing.T) {
+	t.Parallel()
+
+	release := make(chan struct{})
+
+	t.Cleanup(func() { close(release) })
+
+	var wg sync.WaitGroup
+
+	wg.Go(func() { <-release })
+
+	ctx, cancel := context.WithTimeout(
+		context.Background(), waitTimeout,
+	)
+	defer cancel()
+
+	err := lifecycle.WaitForShutdown(
+		ctx, discardLogger(), "test component", &wg,
+	)
+
+	require.ErrorIs(t, err, context.DeadlineExceeded)
+	require.ErrorContains(t, err, "test component")
+}