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/readarr/config.xml

@@ -0,0 +1,17 @@
+<Config>
+  <BindAddress>*</BindAddress>
+  <Port>8787</Port>
+  <SslPort>6868</SslPort>
+  <EnableSsl>False</EnableSsl>
+  <LaunchBrowser>True</LaunchBrowser>
+  <ApiKey>fe0bd24f3efe41bfa43d7b425245f55c</ApiKey>
+  <AuthenticationMethod>None</AuthenticationMethod>
+  <AuthenticationRequired>Enabled</AuthenticationRequired>
+  <LogLevel>debug</LogLevel>
+  <SslCertPath></SslCertPath>
+  <SslCertPassword></SslCertPassword>
+  <UrlBase></UrlBase>
+  <InstanceName>Readarr</InstanceName>
+  <UpdateMechanism>Docker</UpdateMechanism>
+  <Branch>develop</Branch>
+</Config>