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/templates/series-detail.html

@@ -19,6 +19,7 @@
       <input type="number" name="chapter" step="any" placeholder="{{.Chapter}}" required>
       <button type="submit" class="ghost">Set</button>
     </div>
+    {{if .Unverified}}<p class="hint">{{.Unverified}}</p>{{end}}
   </form>
   <form class="dform" hx-post="/admin/series/{{.Key}}/series-url" hx-target="#detail-meta" hx-swap="outerHTML">
     <h3>Repair series URL</h3>
@@ -46,6 +47,7 @@
 {{else}}
 <div class="dform">
   <div class="field"><button type="button" class="ghost" onclick="document.getElementById('confirm-finish').hidden = false">Finish</button></div>
+  {{if .SiteCompleted}}<p class="hint">{{.SiteCompleted}}</p>{{end}}
   <div class="confirm-row calm" id="confirm-finish" role="group" aria-live="polite" hidden>
     <span>Mark this Series finished?</span>
     <div>