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>

deploy/docker-compose.yml

@@ -23,7 +23,7 @@
 
 services:
   selfpost:
-    image: ghcr.io/mixeme/selfpost:2.8.0
+    image: ghcr.io/mixeme/selfpost:2.9.0
     restart: unless-stopped
     environment:
       SELFPOST_HOSTNAME: "${SELFPOST_HOSTNAME:?set the mail/panel hostname, e.g. mail.example.com}"