Fix DMARC guidance for send-only relays with optional rua= settings.

The panel now suggests policy-only DMARC by default, lets operators configure a default and per-domain report address, and DNS-checks hub _report._dmarc records. Future in-panel report ingestion is tracked as dmarc-reports in the roadmap.

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

internal/store/migrations/0004_dmarc_report_email.sql

@@ -0,0 +1,5 @@
+-- Optional DMARC aggregate-report destination (rua=) for the panel administrator
+-- and per-domain overrides. NULL dmarc_rua on a domain means inherit the profile.
+
+ALTER TABLE admin ADD COLUMN dmarc_report_email TEXT NOT NULL DEFAULT '';
+ALTER TABLE domains ADD COLUMN dmarc_rua TEXT;