Add a Readarr mock instance to the demo

Adds darkalex17/readarr-mock, built from ghcr.io/pennydreadful/bookshelf:softcover
(the maintained Readarr continuation; the LinuxServer image is discontinued and
its metadata endpoint no longer resolves). The captured config carries 6 authors,
327 books, 39 book files, 4 tags, 7 indexers, 193 history rows and one root
folder at /books, with minimal EPUBs under mock-htpc/books so the files show as
imported.

Prowlarr now registers Readarr as a synced application, so its config is
recaptured and the prowlarr-mock image must be rebuilt and pushed alongside.

The compose file, managarr config, justfile recipes and README gain the fourth
instance. The demo script needs no change; it fetches the compose file and
config from main at run time.

Requires a Managarr release with Readarr support, since the demo script runs
the published darkalex17/managarr image against these files.

justfile

@@ -20,6 +20,11 @@ default:
 @build-lidarr:
     docker build -f lidarr.Dockerfile -t {{REPO}}/lidarr-mock:{{VERSION}} .
 
+# Build the Readarr mock image
+[group: 'build']
+@build-readarr:
+    docker build -f readarr.Dockerfile -t {{REPO}}/readarr-mock:{{VERSION}} .
+
 # Build the Prowlarr mock image
 [group: 'build']
 @build-prowlarr:
@@ -27,7 +32,7 @@ default:
 
 # Build all mock container images
 [group: 'build']
-@build-all: build-radarr build-sonarr build-lidarr build-prowlarr
+@build-all: build-radarr build-sonarr build-lidarr build-readarr build-prowlarr
 
 # Push the Radarr mock image
 [group: 'push']
@@ -44,6 +49,11 @@ default:
 @push-lidarr: build-lidarr
     docker push {{REPO}}/lidarr-mock:{{VERSION}}
 
+# Push the Readarr mock image
+[group: 'push']
+@push-readarr: build-readarr
+    docker push {{REPO}}/readarr-mock:{{VERSION}}
+
 # Push the Prowlarr mock image
 [group: 'push']
 @push-prowlarr: build-prowlarr
@@ -51,7 +61,7 @@ default:
 
 # Push all mock container images
 [group: 'push']
-@push-all: push-radarr push-sonarr push-lidarr push-prowlarr
+@push-all: push-radarr push-sonarr push-lidarr push-readarr push-prowlarr
 
 # Run the demo
 [group: 'run']