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/latest/read.go

@@ -6,15 +6,19 @@ import (
 	"fmt"
 )
 
-// seriesRead carries the two facts the poll and the acquirer both extract
-// from a series page. Persistence, stamps and scheduling stay with the
-// callers, so the policies that keep the two flows distinct (stamp order,
-// rests) are not swallowed by the module.
+// seriesRead carries the facts the poll and the acquirer both extract from a
+// series page. Persistence, stamps and scheduling stay with the callers, so
+// the policies that keep the two flows distinct (stamp order, rests) are not
+// swallowed by the module.
 type seriesRead struct {
 	Latest    latestChapter
 	HasLatest bool
 	Cover     string
 	HasCover  bool
+	// SiteCompleted is whether the Site's own completed value was on the page.
+	// A challenge body and a redesign both read false — an absent hint, never
+	// a claim (issue #168).
+	SiteCompleted bool
 	// BodyLen is the fetched body's length, surfaced because the no-chapter
 	// log uses it to tell a markup change from a body the size cap cut short.
 	BodyLen int
@@ -25,15 +29,18 @@ type seriesRead struct {
 // errChallengeHeld (browser.go) is the outcome of a Site that answered with
 // its interstitial — status 403 (cf-mitigated) or a challenge page body — and
 // is how a Lane tells a refusal from an ordinary failure (issue #100).
+// errNotFound marks any other 4xx: the page is gone — a fact the owner can act
+// on — as distinct from a Site or database that is merely unwell (issue #164).
 var (
 	errNotFetchable = errors.New("series url not fetchable")
 	errNoFetcher    = errors.New("no fetcher for site")
+	errNotFound     = errors.New("series page not found")
 )
 
 // readSeriesPage performs the series-page read the poll and the acquirer have
 // in common: gate the address, choose the route, fetch the page, extract the
-// Latest Chapter and the Cover address. It persists nothing and stamps
-// nothing.
+// Latest Chapter, the Cover address and the Site's completed value. It
+// persists nothing and stamps nothing.
 //
 // series_url arrives in a client-supplied PUT body (PUT /bookmarks/{key}
 // accepts any string), so the gate is not an optimisation against burning a
@@ -59,6 +66,9 @@ func readSeriesPage(ctx context.Context, site, seriesURL string, browser, tls Fe
 		return seriesRead{}, fmt.Errorf("%w: fetch %s: status %d", errChallengeHeld, seriesURL, status)
 	}
 	if status != 200 {
+		if status >= 400 && status < 500 {
+			return seriesRead{}, fmt.Errorf("%w: fetch %s: status %d", errNotFound, seriesURL, status)
+		}
 		return seriesRead{}, fmt.Errorf("fetch %s: status %d", seriesURL, status)
 	}
 	if isInterstitial(body) {
@@ -69,5 +79,12 @@ func readSeriesPage(ctx context.Context, site, seriesURL string, browser, tls Fe
 	}
 	latest, hasLatest := latestChapterFrom(site, seriesURL, body)
 	cover, hasCover := coverFrom(site, seriesURL, body)
-	return seriesRead{Latest: latest, HasLatest: hasLatest, Cover: cover, HasCover: hasCover, BodyLen: len(body)}, nil
+	return seriesRead{
+		Latest:        latest,
+		HasLatest:     hasLatest,
+		Cover:         cover,
+		HasCover:      hasCover,
+		SiteCompleted: siteCompletedFrom(site, seriesURL, body),
+		BodyLen:       len(body),
+	}, nil
 }