feat: add sequin date-based backfill script and missing backfill indexes

backend/src/osspckgs/migrations/V1784745489__sequin_backfill_sort_column_indexes.sql

@@ -0,0 +1,11 @@
+-- Sequin backfills paginate with a keyset cursor of (sort column, primary key),
+-- e.g. WHERE ("last_synced_at", "id", "package_id") >= ($1, $2, $3) ORDER BY ... LIMIT n.
+-- Without these indexes every page is a full sort (28M+ rows on versions,
+-- 44M+ on package_dependencies) and hits Sequin's per-query timeout.
+-- CONCURRENTLY relies on flyway's -mixed=true to run outside a transaction.
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS versions_last_synced_at_id_package_id_idx
+  ON versions (last_synced_at, id, package_id);
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS package_dependencies_updated_at_id_depends_on_id_idx
+  ON package_dependencies (updated_at, id, depends_on_id);