release: 2.8.0: panel-dkim-signing - the panel signs, OpenDKIM gone

Cut at the owner's word (2026-10-16, "Выпускай").

The panel signs outgoing mail in a milter of its own (go-msgauth,
relaxed/simple, the login's key on 465/587 and the server's name's on
local mail); OpenDKIM, the syslog relay, the USR1 reload and the From:
parser port are gone. Keys move to /data/dkim (copied once; /data/opendkim
kept for a rollback, left out of backups). The sender check holds Sender:
like From:. go-milter is built patched not to keep every header. The queue
is handed to Postfix's users by name before "postfix check". Commits
005922b..ca3a183 (005922b, faa3c11, c2f63d2, f0dd9f9, 2cfd239, d9d9af3,
0cceb54, 80faf1a, f1d90a2, ca3a183).

On the test server since 2026-10-15 22:48 UTC (2.7.0-post.c2f63d2, now
2.7.0-post.f1d90a2, the same code as this cut): healthy, keys unchanged.
The first rollout (faa3c11) crash-looped five minutes on the queue's owner
numbers and was rolled back; fixed in c2f63d2. The owner's canary: Gmail
dkim/spf/dmarc pass with d=carrel.mixfed.ru, Yandex dkim/spf pass, not
spam; his From:/Sender: check, thirteen cases, all as expected.

CHANGELOG [2.8.0]; pins in deploy/docker-compose.yml, README and the guide
(the guide's rollback command keeps naming 2.7.0 on purpose); development's
"from pin"; roadmap row and recommended order; plan stage 3 closed. No tag.

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

docs/guide.md

@@ -204,14 +204,14 @@ 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 `2.7.0`. **No 2.x image is
+deliberately never `:latest`. The current pin is `2.8.0`. **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=2.7.0 \
-  -t ghcr.io/mixeme/selfpost:2.7.0 .
+docker build -f build/Dockerfile --build-arg VERSION=2.8.0 \
+  -t ghcr.io/mixeme/selfpost:2.8.0 .
 ```
 
 `VERSION` must equal the tag: it is the version the backup check compares
@@ -934,7 +934,7 @@ 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 2.6.1 but this image is 2.7.0 — restore into the matching image (selfpost:2.6.1)
+backup: this backup was created by SelfPost 2.7.0 but this image is 2.8.0 — restore into the matching image (selfpost:2.7.0)
 ```
 
 Fix the tag in `docker-compose.yml` (build that tag as under [Fixed image