Remember when a Site called a work completed (#169)

Add series.site_completed_at (epoch-ms, default 0): the durable answer
#168's predicates compute. checkOne writes it after the successful read
on the zero/non-zero transition only, so a Series still completed keeps
its original stamp (the age #170 prints is 'since the Site first said
so'), one that stopped is zeroed, one that became completed is stamped.
The due query projects the column, so the transition is a field
comparison on the snapshot checkOne already holds.

Refused, unreachable and errored reads reach nothing (AC4 is placement,
not a guard); the write sits before the no-chapter and unchanged-number
returns, so a completed page whose chapter did not change still writes.
Store failure logs and carries on — the outcome word never changes.

Series-level like Latest Chapter: a bookmark's updated_at is never
touched. #170 is the surface; nothing reads the column yet.

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
@@ -35,8 +39,8 @@ var (
 
 // 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
@@ -75,5 +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
 }