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>

internal/store/store.go

@@ -1,7 +1,8 @@
 // Package store owns the SelfPost SQLite database: the single file under /data
-// that persists the administrator account, sending domains and applications,
-// the send log and rate-limit settings (architecture.md § Persistence). It
-// exposes typed queries so the rest of the panel never builds SQL by hand.
+// that persists panel users (global administrators and domain-admins), sending
+// domains and applications, the send log and rate-limit settings
+// (architecture.md § Persistence). It exposes typed queries so the rest of the
+// panel never builds SQL by hand.
 package store
 
 import (