release: 2.13.0: local-time - the panel in the container's time zone

No migration; schema 12. The pins (README, compose, guide), the guide's
backup example, the planned cuts and the roadmap row.

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=2.12.0 \
->   -t ghcr.io/mixeme/selfpost:2.12.0 .
+> docker build -f build/Dockerfile --build-arg VERSION=2.13.0 \
+>   -t ghcr.io/mixeme/selfpost:2.13.0 .
 > ```
 
 ```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:2.12.0
+  ghcr.io/mixeme/selfpost:2.13.0
 ```
 
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