feat(db): backfill prefix snapshots and switch read path

Backfill миграция; чтение префиксов через snapshot; запись без дублирования rows.

Co-authored-by: Cursor <cursoragent@cursor.com>

migrations/postgres/000017_prefix_snapshot_backfill.down.sql

@@ -0,0 +1,3 @@
+DELETE FROM prefix_snapshot_row;
+UPDATE config_revision SET prefix_snapshot_id = NULL WHERE prefix_snapshot_id IS NOT NULL;
+DELETE FROM prefix_snapshot;