Merge #169: The pass remembers when a Site called a work completed

# Conflicts:
#	backend/internal/latest/poller_test.go

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;