Record poll failures as rows: the row is the failure state (#165)

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
+);