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/AGENTS.md

@@ -159,6 +159,15 @@ failures. The flag decays and they probe again.
 - Browser Lanes wake Chrome only when 5+ Series are due or one has waited 15m,
   and cover work runs in the background so a slow CDN can't eat a Lane's gap.
 
+### Owner notices — `internal/notify`, `latest.Fault`, `latest.Notifier`, `latest.FaultsFrom`
+
+The poller's outbound owner-notice path (issue #171): one condition today
+(the stall), judged from the durable pass log alone so the poller and any
+future reader of the same judgement cannot disagree. The webhook address is a
+secret in the class of TOKEN_KEY — never logged, never rendered, never
+carried in an error. The `owner_notices` suppression table (one row per
+condition + site) is the only state; every threshold is derived, not stored.
+
 ### Covers — `Store.OnSeriesCreated`, `latest.Acquirer`, `latest.CoverBytesFetcher`, `Store.SetSeriesCover`
 
 Acquired once when the first Bookmark of a Series is created, then served from