test, docs: admin page gate tests and the page's written rules (#102)

- web_test.go walks web.AdminPatterns() rather than naming routes by hand, so a
  new administrative route that forgets requireOwner fails the gate test
  instead of shipping open.
- The harnesses take a LaneReporter; a fake one keeps the page's tests free of
  a poller and a Site.
- backend/AGENTS.md records the adminRoutes/requireOwner rule and the nil-poller
  trap; design-system.md records --patina and the admin page's shape.

backend/api_test.go

@@ -48,7 +48,7 @@ func TestMain(m *testing.M) { os.Exit(pgtest.Main(m)) }
 
 func newTestServer(t *testing.T) http.Handler {
 	t.Helper()
-	return newRouter(newTestStore(t), testConfig())
+	return newRouter(newTestStore(t), testConfig(), nil)
 }
 
 func newTestStore(t *testing.T) *store.Store {
@@ -599,7 +599,7 @@ func TestLoadConfigDiscord(t *testing.T) {
 // cooldown and the poller would re-fetch that series on every single tick.
 func TestPutDoesNotClobberLatestCheckedAt(t *testing.T) {
 	s := newTestStore(t)
-	srv := newRouter(s, testConfig())
+	srv := newRouter(s, testConfig(), nil)
 
 	seedForCheck(t, s, "asura:x", "https://asurascans.com/comics/x", 777)
 
@@ -637,7 +637,7 @@ func TestUserscriptServedWithWebUIDisabled(t *testing.T) {
 
 	rr := httptest.NewRecorder()
 	req := httptest.NewRequest(http.MethodGet, "/u/"+ownerCredential()+"/manga-bookmark.user.js", nil)
-	newRouter(s, cfg).ServeHTTP(rr, req)
+	newRouter(s, cfg, nil).ServeHTTP(rr, req)
 	if rr.Code != http.StatusOK {
 		t.Fatalf("status = %d, want 200", rr.Code)
 	}
@@ -659,7 +659,7 @@ func TestNovelUserscriptServed(t *testing.T) {
 
 	cfg := testConfig()
 	cfg.NovelUserscriptPath = novelPath
-	srv := newRouter(s, cfg)
+	srv := newRouter(s, cfg, nil)
 
 	rr := httptest.NewRecorder()
 	srv.ServeHTTP(rr, httptest.NewRequest(http.MethodGet,