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>

.env.example

@@ -100,7 +100,13 @@ DISCORD_REDIRECT_URI=
 # every kagane poll. Left unset here on purpose — a wrong default would poll a
 # stranger's address, and "no browser" is a safe, self-announcing state.
 # BROWSER_WS_URL=ws://100.x.y.z:9222
-
+#
+# Discord webhook for owner notices (outbound alerting when a poll Lane
+# stalls). Unset means the whole path is off — a local stack needs no webhook,
+# exactly as the browser URL behaves. The address is a secret in the class of
+# TOKEN_KEY: never commit it, never paste it anywhere public.
+# DISCORD_WEBHOOK_URL=https://discord.com/api/webhooks/...
+#
 # Zone the backend stamps its log lines in. Cosmetic only. Nothing else in
 # the service has a zone: bookmark timestamps are unix ms, and the two real
 # time columns are timestamptz. Defaults to Asia/Jakarta; set to UTC for the