Owner-only admin page: Reader roster plus Poll Lane status (#102) (#107)

Closes #102.

The only operational surface was /healthz and a fold-out roster inside the owner's own reading page. This adds /admin: an owner-only page carrying the Reader roster and one row per Poll Lane.

- **Poller seam.** `latest.Poller` records each Lane's last pass (`Site`, `Due`, `Checked`, `LastRun`, `Gap`, `Clamped`, `Browser`) and answers `LaneStatus()`; the page reads that snapshot, never a table. A pass that returns before computing its figures (refusal backoff, sidecar down) carries the previous pass's figures forward rather than recording zeroes, and a Lane that has never reached a pace renders no gap at all. Refusal and sidecar reachability are derived at snapshot time.
- **Owner gate at registration.** Every route reaching past the acting Reader lives in `adminRoutes()` and is wrapped in `requireOwner` when it is registered, so a missing gate is visible in the route list rather than hidden in a handler. `web.AdminPatterns()` is what the gate test walks, so a new route cannot be added without being tested. A non-owner gets 404, never 403.
- **Nil poller is a first-class state.** `main.newRouter` takes the reporter as an interface and converts a nil `*Poller` to a nil interface; no poller and no completed pass both render "No data yet" with the reason spelled out, rather than confident zeroes.
- **Roster moved** off the reading page onto /admin, with the Sighting counters and a confirm-gated `Clear marks` control. #103 fills those counters, so on delivery they read zero for everyone - deliberate ordering.
- **One accent, `--patina`** (verdigris, both colour branches): the far side of the wheel from ember's crimson and clear of the archive blue. Ember still means new chapter only; revocation still wears --danger.

Verification: `go vet ./...` and `go test ./...` green (Docker-backed); admin page screenshotted at 1100px and 390px in both colour schemes. Reviewed on both axes (spec, standards); findings on the accent hue, zero-figure honesty and three tests that could not fail are fixed in 58014eb.
Reviewed-on: #107
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

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,