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/store/migrations/0020_owner_notices.sql

@@ -0,0 +1,12 @@
+-- Owner notices (issue #171): one row per episode, remembered only as "the
+-- owner was told". The row's presence is the whole state — the poller checks
+-- it before sending, writes it after a successful send, and clears it when
+-- the condition no longer holds. site is '' for a fault that is not one
+-- Site's; the composite primary key is what makes the row a lock against a
+-- second message for the same episode.
+CREATE TABLE owner_notices (
+	condition text NOT NULL,
+	site text NOT NULL,
+	notified_at bigint NOT NULL,
+	PRIMARY KEY (condition, site)
+);