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>

docker-compose.yml

@@ -75,7 +75,11 @@ services:
       # Host header is an IP address or "localhost" — confirmed 2026-08-03,
       # independent of chromedp's own dial logic. The same trap that used to
       # force a pinned Docker IP now forbids the tailnet name.
-      BROWSER_WS_URL: ${BROWSER_WS_URL:-}
+      # Owner-notice webhook (issue #171). Empty default, never a
+      # required-guard: unset means the whole path is off, so a local stack
+      # runs exactly as it does today. An env var not listed here never
+      # reaches the container.
+      DISCORD_WEBHOOK_URL: ${DISCORD_WEBHOOK_URL:-}
     depends_on:
       # The migration runner is the first thing the binary does, so a Postgres
       # that is still initialising means a crash-loop until it is not.