feat(store): persist per-domain custom record marks

internal/store/migrations/0005_domain_custom_records.sql

@@ -0,0 +1,11 @@
+-- +goose Up
+CREATE TABLE domain_custom_records (
+    domain_id  uuid NOT NULL REFERENCES domains(id) ON DELETE CASCADE,
+    record_key text NOT NULL,
+    note       text NOT NULL DEFAULT '',
+    created_at timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (domain_id, record_key)
+);
+
+-- +goose Down
+DROP TABLE domain_custom_records;