release: 2.7.0: base-trixie - Debian trixie, Postfix 3.10.13

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

The image runs debian:trixie-slim with Postfix 3.10.13, libmilter
8.18.1, OpenSSL 3.5; every citation re-read against the trixie sources.
smtpd_hide_client_session=yes on 465/587 (owner's decision). The
selfpost group is 10998 and the panel 10999:10999, outside the system
range; the entrypoint re-owns /data by name. The panel's database is
0600; the entrypoint removes symbolic links the panel could leave under
/data. Commits 68ec8b2, 1c6e04b, a3050af.

Rolled out to the test server as 2.6.1-post.a3050af (the same code),
2026-10-15 17:07 UTC: healthy, every program RUNNING, no restarts, three
messages sent, the queue empty, no warning but outside password guessing.
The canary: one message from an application through 465 to Gmail, Yandex
and the test mailbox - Received: in the short form everywhere; Gmail
dkim=pass spf=pass dmarc=pass; Yandex spf=pass dkim=pass, not spam.
No migration, schema 11.

CHANGELOG cut; pin 2.7.0 in deploy/docker-compose.yml, README, guide (and
its sample restore message); development.md "from pin" and the rows of
base-trixie and panel-dkim-signing; roadmap row and recommended order.
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 `2.6.1`. **No 2.x image is
+deliberately never `:latest`. The current pin is `2.7.0`. **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=2.6.1 \
-  -t ghcr.io/mixeme/selfpost:2.6.1 .
+docker build -f build/Dockerfile --build-arg VERSION=2.7.0 \
+  -t ghcr.io/mixeme/selfpost:2.7.0 .
 ```
 
 `VERSION` must equal the tag: it is the version the backup check compares
@@ -874,7 +874,7 @@ 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 2.6.0 but this image is 2.6.1 — restore into the matching image (selfpost:2.6.0)
+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)
 ```
 
 Fix the tag in `docker-compose.yml` (build that tag as under [Fixed image