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.

mock-htpc/managarr/config.yml

@@ -14,3 +14,8 @@ lidarr:
     host: lidarr-mock
     port: 8686
     api_token: 611b3863baa2442f975afbc8f1377037
+readarr:
+  - name: Books
+    host: readarr-mock
+    port: 8787
+    api_token: fe0bd24f3efe41bfa43d7b425245f55c