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.up.sql

@@ -0,0 +1,7 @@
+CREATE TABLE asn_prefix_cache_row (
+    asn     INTEGER NOT NULL REFERENCES asn_prefix_cache (asn) ON DELETE CASCADE,
+    prefix  TEXT NOT NULL,
+    PRIMARY KEY (asn, prefix)
+);
+
+ALTER TABLE asn_prefix_cache DROP COLUMN prefixes_json;