release: 2.4.1: code audit of 2.4.0 and its plan

Docs only. docs/audit-2026-10-12.md: findings of a full-tree audit (four
high, twenty-one medium, the rest), doc drift, test gaps, what was checked
and found sound, and a six-stage plan; roadmap index row; CHANGELOG cut and
pin bump. Old tags and images stay as they are (owner): fixes go forward.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

deploy/docker-compose.yml

@@ -21,7 +21,7 @@
 
 services:
   selfpost:
-    image: ghcr.io/mixeme/selfpost:2.4.0
+    image: ghcr.io/mixeme/selfpost:2.4.1
     restart: unless-stopped
     environment:
       SELFPOST_HOSTNAME: "${SELFPOST_HOSTNAME:?set the mail/panel hostname, e.g. mail.example.com}"