release: 2.13.1: docs-audit - the documents and the panel's words against the code

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

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/guide.md

@@ -205,14 +205,14 @@ 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 `2.13.0`. **No 2.x image is
+deliberately never `:latest`. The current pin is `2.13.1`. **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=2.13.0 \
-  -t ghcr.io/mixeme/selfpost:2.13.0 .
+docker build -f build/Dockerfile --build-arg VERSION=2.13.1 \
+  -t ghcr.io/mixeme/selfpost:2.13.1 .
 ```
 
 `VERSION` must equal the tag: it is the version the backup check compares
@@ -989,7 +989,7 @@ 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 2.12.0 but this image is 2.13.0 — restore into the matching image (selfpost:2.12.0)
+backup: this backup was created by SelfPost 2.13.0 but this image is 2.13.1 — restore into the matching image (selfpost:2.13.0)
 ```
 
 Fix the tag in `docker-compose.yml` (build that tag as under [Fixed image