WIP: root background loops at context.Background() (refs #97)

Incomplete: tests pass but three lint findings remain (funcorder on
registerHooks, unparam in engine_integration_test.go). Committed to
preserve work in progress; to be amended into a single clean commit.

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)