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>

docs/guide.md

@@ -190,14 +190,14 @@ cat ./data/setup-token
 #### Fixed image tag
 
 `deploy/docker-compose.yml` pins an explicit version (`ghcr.io/mixeme/selfpost:X.Y.Z`),
-deliberately never `:latest`. The current pin is `2.5.4`. **No 2.x image is
+deliberately never `:latest`. The current pin is `2.6.0`. **No 2.x image is
 published on ghcr.io yet** (the newest there is `1.3.0`); until one is, build
 it from the checkout under the pinned name, and Compose uses the local image
 instead of pulling:
 
 ```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 .
 ```
 
 `VERSION` must equal the tag: it is the version the backup check compares
@@ -861,7 +861,7 @@ A version mismatch at step 5 refuses to start and leaves `/data` untouched —
 the panel exits with a message naming the tag to use, e.g.:
 
 ```
-backup: this backup was created by SelfPost 2.5.3 but this image is 2.5.4 — restore into the matching image (selfpost:2.5.3)
+backup: this backup was created by SelfPost 2.5.4 but this image is 2.6.0 — restore into the matching image (selfpost:2.5.4)
 ```
 
 Fix the tag in `docker-compose.yml` (build that tag as under [Fixed image