Root background loops at context.Background() (closes #97) (#100)

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.

internal/database/retention.go

@@ -56,9 +56,20 @@ func NewRetentionReaper(
 		interval:  params.Config.RetentionSweepInterval,
 	}
 
+	r.registerHooks(lc)
+
+	return r
+}
+
+// registerHooks wires the reaper's start and stop into the fx
+// lifecycle. The start hook's context is deliberately ignored: see
+// start for why the sweep loop must not inherit it.
+func (r *RetentionReaper) registerHooks(lc fx.Lifecycle) {
 	lc.Append(fx.Hook{
-		OnStart: func(ctx context.Context) error {
-			r.start(ctx)
+		//nolint:contextcheck // Not inheriting the hook context is
+		// the point: see start.
+		OnStart: func(_ context.Context) error {
+			r.start()
 
 			return nil
 		},
@@ -68,12 +79,20 @@ func NewRetentionReaper(
 			return nil
 		},
 	})
-
-	return r
 }
 
-func (r *RetentionReaper) start(ctx context.Context) {
-	ctx, cancel := context.WithCancel(ctx)
+// start launches the background sweep loop.
+//
+// The loop's context is derived from context.Background(), NOT from
+// the fx OnStart hook context. The hook context carries fx's start
+// timeout (15s by default) and is cancelled once the start phase
+// completes, so a loop derived from it dies 45 minutes before its
+// first tick under the default one-hour sweep interval, leaving a
+// reaper that never reaps. A long-lived goroutine must outlive the
+// startup phase, so its lifetime is bounded by OnStop instead: stop
+// cancels this context and waits on the WaitGroup.
+func (r *RetentionReaper) start() {
+	ctx, cancel := context.WithCancel(context.Background())
 	r.cancel = cancel
 
 	r.wg.Add(1)