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/delivery/archive_sweeper.go

@@ -10,6 +10,7 @@ import (
 	"go.uber.org/fx"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/lifecycle"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
@@ -67,10 +68,9 @@ func NewArchiveSweeper(
 }
 
 // registerHooks wires the sweeper's start and stop into the fx
-// lifecycle. Both hook contexts are deliberately ignored: see
-// start for why the background loop must not inherit the start
-// hook's context, and stop for why shutdown blocks on the loop
-// rather than on the stop hook's deadline.
+// lifecycle. The start hook's context is deliberately ignored
+// (see start for why the background loop must not inherit it);
+// the stop hook's context is honoured (see stop).
 func (s *ArchiveSweeper) registerHooks(lc fx.Lifecycle) {
 	lc.Append(fx.Hook{
 		//nolint:contextcheck // Not passing the hook context is
@@ -80,10 +80,8 @@ func (s *ArchiveSweeper) registerHooks(lc fx.Lifecycle) {
 
 			return nil
 		},
-		OnStop: func(_ context.Context) error {
-			s.stop()
-
-			return nil
+		OnStop: func(ctx context.Context) error {
+			return s.stop(ctx)
 		},
 	})
 }
@@ -113,15 +111,27 @@ func (s *ArchiveSweeper) start() {
 	)
 }
 
-func (s *ArchiveSweeper) stop() {
+// stop cancels the sweep loop's context and waits for it to
+// exit, bounded by the stop hook's context: a prune wedged on a
+// locked archive must not hang the process past fx's stop
+// timeout.
+func (s *ArchiveSweeper) stop(ctx context.Context) error {
 	s.log.Info("archive sweeper stopping")
 
 	if s.cancel != nil {
 		s.cancel()
 	}
 
-	s.wg.Wait()
+	err := lifecycle.WaitForShutdown(
+		ctx, s.log, "archive sweeper", &s.wg,
+	)
+	if err != nil {
+		return err
+	}
+
 	s.log.Info("archive sweeper stopped")
+
+	return nil
 }
 
 func (s *ArchiveSweeper) run(ctx context.Context) {