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.

docker-compose.yml

@@ -15,6 +15,11 @@ services:
     container_name: lidarr-mock
     restart: unless-stopped
 
+  readarr-mock:
+    image: darkalex17/readarr-mock:latest
+    container_name: readarr-mock
+    restart: unless-stopped
+
   prowlarr-mock:
     image: darkalex17/prowlarr-mock:latest
     container_name: prowlarr-mock
@@ -33,5 +38,7 @@ services:
         condition: service_started
       lidarr-mock:
         condition: service_started
+      readarr-mock:
+        condition: service_started
       prowlarr-mock:
         condition: service_started