Add a table to record where narinfo files were fetched from

Otherwise it's hard to associated narinfo files to build servers.

sqitch/deploy/narinfo_fetch_record.sql

@@ -0,0 +1,12 @@
+-- Deploy guix-data-service:narinfo_fetch_record to pg
+
+BEGIN;
+
+CREATE TABLE narinfo_fetch_records (
+    id integer PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
+    narinfo_signature_data_id integer NOT NULL REFERENCES narinfo_signature_data(id),
+    build_server_id integer NOT NULL REFERENCES build_servers(id),
+    fetched_at timestamp without time zone DEFAULT clock_timestamp()
+);
+
+COMMIT;