refactor(db): normalize asn prefix cache and ttl cleanup

Строки asn_prefix_cache_row; периодический prune через evobgp-ingest.

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

migrations/sqlite/000020_asn_cache_rows.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE asn_prefix_cache ADD COLUMN prefixes_json TEXT NOT NULL DEFAULT '[]';
+DROP TABLE IF EXISTS asn_prefix_cache_row;