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.

README.md

@@ -35,6 +35,7 @@ Available recipes:
     build-lidarr   # Build the Lidarr mock image
     build-prowlarr # Build the Prowlarr mock image
     build-radarr   # Build the Radarr mock image
+    build-readarr  # Build the Readarr mock image
     build-sonarr   # Build the Sonarr mock image
 
     ...
@@ -55,6 +56,7 @@ Available recipes:
     push-lidarr    # Push the Lidarr mock image
     push-prowlarr  # Push the Prowlarr mock image
     push-radarr    # Push the Radarr mock image
+    push-readarr   # Push the Readarr mock image
     push-sonarr    # Push the Sonarr mock image
 
     ...