release: 2.6.0: audit stage 5 - image and Postfix hardening

Cut without asking, as the owner said on 2026-10-15.

The supervisord socket is root:supervisor, the panel the group's one member
(M17). Postfix takes TLS 1.2 at least, the client too where TLS is
required; VRFY off; the banner without the distribution; the start refuses
a rate, size or milter timeout that is not a number, under its minimum or
past an int (L31). OpenDKIM logs through the syslog relay to the container
log and oversigns From: (L30). crashexit takes the container down after
five unexpected exits of a mail-path program in ten minutes (L32).
logrotate keeps its state under /data/log. extract-cert.sh refuses what is
not PEM and renames whole files (M21). Plus the findings of both reviews
(a39eda5). Commits dab7715, a39eda5, b09e525.

Rolled out to the test server as 2.5.4-post.95af363 (the same code): the
container healthy, every program RUNNING, syslog-relay among them;
OpenDKIM's start line in docker logs under "syslog: "; the postconf values
as set; /run/supervisor.sock root:supervisor 0770 and the panel reaches it;
/data/log/.logrotate.status written. No migration, schema 11.

CHANGELOG cut; pin 2.6.0 in deploy/docker-compose.yml, README, guide (and
its sample restore message); development.md "from pin" and the stage's row;
roadmap row and recommended order; stage 5 marked shipped in the plan. No
tag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -112,8 +112,8 @@ outbound mail will not reach the real internet without DNS, PTR, and port 25.
 > local image and pulls nothing:
 >
 > ```sh
-> docker build -f build/Dockerfile --build-arg VERSION=2.5.4 \
->   -t ghcr.io/mixeme/selfpost:2.5.4 .
+> docker build -f build/Dockerfile --build-arg VERSION=2.6.0 \
+>   -t ghcr.io/mixeme/selfpost:2.6.0 .
 > ```
 
 ```sh
@@ -122,7 +122,7 @@ docker run --rm -d --name selfpost-try \
   -e SELFPOST_HOSTNAME=mail.local.test \
   -e PANEL_COOKIE_SECURE=false \
   -v selfpost-try-data:/data \
-  ghcr.io/mixeme/selfpost:2.5.4
+  ghcr.io/mixeme/selfpost:2.6.0
 ```
 
 **Get the setup URL** (pick one):