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/0017_poll_passes_not_found.sql

@@ -0,0 +1,4 @@
+-- A 4xx other than the 403 refusal is the page being gone, not the Site being
+-- unwell; the pass log counts it separately so the Lanes page can say "not
+-- found" (#164). DEFAULT 0 keeps pre-existing rows readable.
+ALTER TABLE poll_passes ADD COLUMN not_found integer NOT NULL DEFAULT 0;

backend/internal/store/migrations/0018_poll_failures.sql

@@ -0,0 +1,13 @@
+-- One row per Series that is failing right now (ADR-0016): the row's
+-- existence is the failure state, failing_since ages the run of failures,
+-- and a correct read deletes the row. Keyed by the same (site, series_id)
+-- composite the rest of the system uses, with the cascade so deleting a
+-- Series takes its failure row and orphan removal stays a single statement.
+CREATE TABLE poll_failures (
+	site text NOT NULL,
+	series_id text NOT NULL,
+	outcome text NOT NULL,
+	failing_since bigint NOT NULL,
+	PRIMARY KEY (site, series_id),
+	FOREIGN KEY (site, series_id) REFERENCES series (site, series_id) ON DELETE CASCADE
+);

backend/internal/store/migrations/0019_series_site_completed.sql

@@ -0,0 +1,4 @@
+-- When the last successful Poll read saw the Site's own completed value
+-- (#168): epoch-ms, zero meaning it did not. DEFAULT 0 keeps pre-existing
+-- rows readable.
+ALTER TABLE series ADD COLUMN site_completed_at bigint NOT NULL DEFAULT 0;

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)
+);