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/prowlarr/asp/key-f35075e6-73f1-4900-9906-cba5b6f0af24.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<key id="f35075e6-73f1-4900-9906-cba5b6f0af24" version="1">
+  <creationDate>2026-09-11T16:08:36.9329939Z</creationDate>
+  <activationDate>2026-09-11T16:08:36.9241052Z</activationDate>
+  <expirationDate>2026-12-10T16:08:36.9241052Z</expirationDate>
+  <descriptor deserializerType="Microsoft.AspNetCore.DataProtection.AuthenticatedEncryption.ConfigurationModel.AuthenticatedEncryptorDescriptorDeserializer, Microsoft.AspNetCore.DataProtection, Version=8.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60">
+    <descriptor>
+      <encryption algorithm="AES_256_CBC" />
+      <validation algorithm="HMACSHA256" />
+      <masterKey p4:requiresEncryption="true" xmlns:p4="http://schemas.asp.net/2015/03/dataProtection">
+        <!-- Warning: the key below is in an unencrypted form. -->
+        <value>p1XbREo2gsuNFreuB5dMUt/vEcyHLuEMPa7ZUs3p8yUXC9iwGAotEyBsIxhjAcAtJBvvWxbdaEyrxu+wxqKQcA==</value>
+      </masterKey>
+    </descriptor>
+  </descriptor>
+</key>