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.go

@@ -4,14 +4,16 @@ import (
 	"log"
 	"net/http"
 	"strconv"
-	"time"
 
+	"bookmarkmanager/backend/internal/latest"
 	"bookmarkmanager/backend/internal/store"
 )
 
 // ownerWindow is the staleness boundary the Series list's "not checked in
-// 12h" filter compares against. Declared once; later admin tickets read it.
-const ownerWindow = 12 * time.Hour
+// 12h" filter compares against. It reads latest.OwnerWindow — the one place
+// the class-level twelve hours lives, shared with the owner-notice
+// conditions (issue #171).
+const ownerWindow = latest.OwnerWindow
 
 // adminView is the shared shell data for an administrative page and the roster
 // fragment returned after a Reader action.