web: site-completed hint reaches the owner, decides nothing (#170)

The Site's own completed marker (issue #168) joins the owner's surface as
an informational hint, never a decision:

- store: SeriesFilterSiteCompleted ('site_completed') sits beside Finished
  in the filter vocabulary; its predicate carries the same finished_at
  guard as the clock-driven filters, because the Site's stamp keeps
  standing after the owner retires the row. The projection and SeriesPage
  GROUP BY carry site_completed_at so the detail page can age it.
- web: the filter joins the Series list select as 'Site says completed'
  and rides the tail of seriesFilterOrder, so the landing stats figure
  comes for free and sits after the finished figure.
- detail: one hint line beside the Finish control - 'the site says this
  work is completed (since 3d ago)' - rendered only while the stamp
  stands and no Finish stands; the Finish control itself is untouched.
- tests: filter's finished exclusion and un-finish return to the Poll
  query (store); figure at zero and nonzero, hint present/absent, and the
  Finish control's markup byte-identical with a hint present (web).

backend/internal/web/admin_series_detail.go

@@ -64,6 +64,10 @@ 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
+	// 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 +155,9 @@ 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)
+	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