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/internal/web/admin_series_detail.go

@@ -64,6 +64,15 @@ type seriesDetailView struct {
 	// stands. It rides the meta fragment both presses swap, so the answer
 	// itself shows how long the Series has been finished.
 	FinishedSince string
+	// Unverified is the sentence beside the Latest Chapter correction
+	// control while a Reader's number stands behind a failure past the
+	// owner window: the value is unconfirmed and the owner should not trust
+	// it. It never names the Reader. "" otherwise.
+	Unverified string
+	// SiteCompleted is the hint's line, "" while the Site has said nothing or
+	// the owner has finished the Series: "the site says this work is
+	// completed (since 3d ago)". A hint, never a control (issue #170).
+	SiteCompleted string
 }
 
 // adminSeriesDetail renders one Series' page, keyed by the composite
@@ -151,6 +160,17 @@ func (h *Handler) seriesDetailView(a store.AdminSeries) seriesDetailView {
 	v.Corrected = correctedAge(time.Now(), a.LatestCorrectedAt)
 	v.Finished = a.FinishedAt != 0
 	v.FinishedSince = finishedAge(time.Now(), a.FinishedAt)
+	// Unverified: a Reader's number standing behind a failure that has outlived
+	// the owner window is a number nobody has re-checked since — say so next to
+	// the correction control, without naming the Reader. A machine read or a
+	// failure still inside the window carries no sentence; the failure itself
+	// has not yet outlived the twelve hours' worth of trust.
+	if a.RaisedByReader && a.FailureOutcome != "" && a.FailingSince < time.Now().Add(-ownerWindow).UnixMilli() {
+		v.Unverified = "Unverified Reader number: the page has been failing for over 12h, so this Reader-reported chapter is unconfirmed."
+	}
+	if a.SiteCompletedAt != 0 && a.FinishedAt == 0 {
+		v.SiteCompleted = "the site says this work is completed (since " + checkedAge(time.Now(), a.SiteCompletedAt) + ")"
+	}
 
 	// Provenance: the actor class behind the current number, evaluated in the
 	// order the classes outrank one another — the owner's stamp, which a