Add Kavita as the new book server

Runs next to Calibre-Web at a temporary kavita. subdomain until it takes
over books. Each book lives in its own folder under
/opt/stacks/kavita/books, because Kavita names a book after its folder.
The admin password is in the vault as vault_kavita.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/docker/tasks/media/main.yml

@@ -9,6 +9,10 @@
   import_tasks: calibre.yml
   tags: calibre
 
+- name: Install kavita
+  import_tasks: kavita.yml
+  tags: kavita
+