release: 2.5.1: audit stage 2b - the log tailer and what the panel shows of a refusal

A log rotated while the panel was down is read to its end; the deferred
counter remembers a place in the log, not a second; a refused message's
page shows what Postfix wrote; a refusal the panel did not get to write is
written from mail.log; downtime is estimated (≈) when supervisord is
silent; the Sender check card counts connections and messages apart (the
owner's decision); a message the sweep closed says the panel closed it.
Plus the findings of both reviews of the stage.

Rolled out to the test server as 2.5.0-post.e1ddf12 (the same code):
migration 0009 ran on the live database - schema 9, the new columns in
place, 68/68 send-log rows still tied to their domain, integrity ok.

CHANGELOG cut; pin 2.5.1 in deploy/docker-compose.yml, README (with the
local-build note), guide; development.md "from pin"; roadmap row and the
recommended order; stage 2b marked shipped in the plan. 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=2.5.0 \
->   -t ghcr.io/mixeme/selfpost:2.5.0 .
+> docker build -f build/Dockerfile --build-arg VERSION=2.5.1 \
+>   -t ghcr.io/mixeme/selfpost:2.5.1 .
 > ```
 
 ```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:2.5.0
+  ghcr.io/mixeme/selfpost:2.5.1
 ```
 
 **Get the setup URL** (pick one):