docs: rewrite security/operator docs for the multi-user panel (P2)

The CSRF ADR, guide.md, architecture.md route table, the internal/backup
package comment, roadmap/dmarc-reports admin.dmarc_report_email references,
the e2e coverage summary, and the setupManager/store package comments still
described the pre-1.2.0 single-administrator panel or stale implementation
details. Closes P2 of docs/plans/code-review.md; no behaviour change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

docs/plans/dmarc-reports.md

@@ -18,7 +18,8 @@ operator.
 - gzip + XML aggregate parsing → SQLite summaries per sending domain.
 - Panel page and/or per-domain section: recent reports, third-party senders,
   delivery health of report ingestion.
-- Reuse `admin.dmarc_report_email` and `domains.dmarc_rua` for DNS templates;
+- Reuse the `dmarc_report_email` setting (moved off the old `admin` table into
+  `settings` by migration `0005`) and `domains.dmarc_rua` for DNS templates;
   when enabled, suggest a SelfPost-hosted report address.
 
 **Out:**