Say what each action did in a one-line notice (closes #383)

Saving, deleting, activating or deactivating a webhook, entrypoint or target, and signing out, now land on their page with a one-line notice such as "Webhook deleted." or "Signed out.". The redirect carries a fixed code that maps to fixed text; an unknown code shows nothing, so nothing from the URL is ever echoed. One partial in the page layout shows the notice on every page, and replay and resubmit now use the same codes and partial. Error pages show no notice.

Model: opus-5-5

internal/handlers/event_resubmit_test.go

@@ -154,7 +154,7 @@ func TestHandleEventResubmit_DeliversToTargetCreatedAfterTheEvent(
 	require.Equal(t, http.StatusSeeOther, w.Code)
 	assert.Equal(
 		t,
-		"/hook/"+wh.ID+"/events?resubmit=queued",
+		"/hook/"+wh.ID+"/events?notice=resubmit-queued",
 		w.Header().Get("Location"),
 	)
 
@@ -282,7 +282,7 @@ func TestHandleEventResubmit_IsRepeatable(t *testing.T) {
 		require.Equal(t, http.StatusSeeOther, w.Code)
 		assert.Equal(
 			t,
-			"/hook/"+wh.ID+"/events?resubmit=queued",
+			"/hook/"+wh.ID+"/events?notice=resubmit-queued",
 			w.Header().Get("Location"),
 			"a resubmit must not be refused while an earlier "+
 				"one is in flight",
@@ -436,7 +436,7 @@ func TestHandleEventResubmit_SkipsInactiveTarget(t *testing.T) {
 	require.Equal(t, http.StatusSeeOther, w.Code)
 	assert.Equal(
 		t,
-		"/hook/"+wh.ID+"/events?resubmit=queued",
+		"/hook/"+wh.ID+"/events?notice=resubmit-queued",
 		w.Header().Get("Location"),
 		"an inactive target is skipped, not an error",
 	)
@@ -482,7 +482,7 @@ func TestHandleEventResubmit_NoActiveTargetsStillStoresEvent(
 	require.Equal(t, http.StatusSeeOther, w.Code)
 	assert.Equal(
 		t,
-		"/hook/"+wh.ID+"/events?resubmit=no-targets",
+		"/hook/"+wh.ID+"/events?notice=resubmit-no-targets",
 		w.Header().Get("Location"),
 	)