fix(19.3-06): queue site-cache purges after commit and coalesce them (WR-01)

- PurgeSiteCacheAfterCommit queues the purge on a per-purger worker after
  commit: one request in flight, saves committed meanwhile collapse into
  one follow-up, so the admin request never waits for the site
- new cooler.SiteCachePurger.Wait drains the queue; Handle.Close waits for
  it at shutdown (timeout plus one second)
- Purge stays the synchronous send; exported names and signatures unchanged

modules/cooler/example_test.go

@@ -59,9 +59,9 @@ type articlesController struct {
 	app func() *backpack.App
 }
 
-// purge registers one site-cache purge for the saved article, sent after the
-// save transaction commits (at once outside one). Without a purge configured
-// the purger is nil and nothing is sent.
+// purge queues one site-cache purge for the saved article once the save
+// transaction commits (at once outside one); the save never waits for the
+// site. Without a purge configured the purger is nil and nothing is sent.
 func (c articlesController) purge(ctx context.Context, model any) error {
 	article, ok := model.(*Article)
 	if !ok {
@@ -121,5 +121,9 @@ func ExampleSiteCacheFromApp() {
 	if err := c.FormAfterUpdate(context.Background(), &Article{Slug: "hello-summer"}); err != nil {
 		fmt.Println(err)
 	}
+	// The purge is queued; Wait drains the queue (serve does it at shutdown).
+	if err := cooler.SiteCacheFromApp(app).Wait(context.Background()); err != nil {
+		fmt.Println(err)
+	}
 	// Output: POST /api/__site__/cache/purge {"kind":"article","slug":"hello-summer"}
 }