Spec #137: per-Series poll failure state, completion hint, and outbound owner notification (#174)

Implements spec #137 (spec 4 of 4 from wayfinder map #114).

Closes #137.

Tickets: #164, #165, #166, #167, #168, #169, #170, #171, #172, #173 — all closed, landed on this branch.

## Summary
- #164/#168: sixth outcome word `not_found`; per-Site completed marker predicate.
- #165/#169: `poll_failures` row is the failure state; the pass remembers a Site-reported completion.
- #166/#171: two new admin filters (`failing`, `unverified`); outbound owner notification + stall condition.
- #167/#170: a failure names itself on the Series page; the completion hint reaches the owner and decides nothing.
- #172: the other three fault conditions (no-browser-route, sidecar-down, adapter-broken) feeding the notifier.
- #173: the landing verdict line shares the same `latest.FaultsFrom` judgement the notifier uses, so the page and the push cannot disagree.

`cd backend && go test ./...` green on the merged branch (8 packages).

Reviewed-on: #174
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/main_test.go

@@ -28,6 +28,20 @@ func TestLoadConfigReadsCoverDirectory(t *testing.T) {
 	}
 }
 
+func TestLoadConfigReadsDiscordWebhook(t *testing.T) {
+	// The address is read, never defaulted: unset stays empty (the whole
+	// path is off), set flows into the Config for the poller's notifier.
+	const url = "https://discord.com/api/webhooks/000000/secret"
+	t.Setenv("DISCORD_WEBHOOK_URL", url)
+	if got := loadConfig().DiscordWebhookURL; got != url {
+		t.Fatalf("DiscordWebhookURL = %q, want %q", got, url)
+	}
+	t.Setenv("DISCORD_WEBHOOK_URL", "")
+	if got := loadConfig().DiscordWebhookURL; got != "" {
+		t.Fatalf("DiscordWebhookURL = %q, want empty when unset", got)
+	}
+}
+
 func TestLoadLatestPollEnabledParsing(t *testing.T) {
 	tests := []struct {
 		raw  string
@@ -51,7 +65,8 @@ func TestLoadLatestPollEnabledParsing(t *testing.T) {
 // nothing here sizes a cooldown any more.
 func TestNewLatestPollerWiresFetchers(t *testing.T) {
 	tls := &latest.TLSFetcher{}
-	p := newLatestPoller(nil, LatestPoll{Enabled: true}, tls, nil)
+	notifier := &stubNotifier{}
+	p := newLatestPoller(nil, LatestPoll{Enabled: true}, tls, nil, notifier)
 	if p.Fetch != tls {
 		t.Fatalf("Fetch not wired")
 	}
@@ -67,8 +82,15 @@ func TestNewLatestPollerWiresFetchers(t *testing.T) {
 	if p.Now == nil {
 		t.Fatalf("Now = nil, want the live clock")
 	}
+	if p.Notify != notifier {
+		t.Fatalf("Notify = %v, want the configured notifier", p.Notify)
+	}
 }
 
+// stubNotifier satisfies latest.Notifier so newLatestPoller's wiring can be
+// asserted; it is never called.
+type stubNotifier struct{ latest.Notifier }
+
 func TestPutStatusValidation(t *testing.T) {
 	cases := []struct {
 		name   string