release: 2.9.0: DNS records folded, Settings re-checks, /data/opendkim gone

Cut at the owner's word (2026-10-16, "потом режь 2.9.0").

dns-record-collapse (fffee9c, 51f303c): a record DNS holds as shown is one
line, its values behind Show values. settings-save-row (f95c553,
82a53e1): the Settings form is the left column, Save under its fields.
notification-mail-recheck (98e4dfc, 52a9811, 3f5cd9a): Re-check DNS on
Settings, POST /server/settings/dns-recheck. And the release after 2.8.0
removes /data/opendkim (9420d8c): the entrypoint deletes the 2.7.x key
tree once /data/dkim holds a key, and stops on a /data that never ran
2.8.x.

Each tree passed go vet, go test (26 packages) and make e2e on the test
server; dns-record-collapse and settings-save-row ran there as
2.8.0-post.51f303c and 2.8.0-post.82a53e1. No migration, schema 11.

CHANGELOG [2.9.0]; pins in deploy/docker-compose.yml, README and the guide
(pin, build, the sample restore message; the 2.7.x rollback command keeps
naming 2.7.0); development's "from pin" and row 30; roadmap rows and
recommended order. 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=2.8.0 \
->   -t ghcr.io/mixeme/selfpost:2.8.0 .
+> docker build -f build/Dockerfile --build-arg VERSION=2.9.0 \
+>   -t ghcr.io/mixeme/selfpost:2.9.0 .
 > ```
 
 ```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:2.8.0
+  ghcr.io/mixeme/selfpost:2.9.0
 ```
 
 **Get the setup URL** (pick one):