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>

group_vars/all/domains.yml

@@ -13,6 +13,7 @@ subdomains:
   media: "media.{{ primary_domain }}"
   audio: "audio.{{ primary_domain }}"  # Audiobookshelf
   books: "books.{{ primary_domain }}"  # Calibre
+  kavita: "kavita.{{ primary_domain }}"  # Kavita (temporary, moves to books)
   models: "models.{{ primary_domain }}"  # Manyfold
   paper: "paper.{{ primary_domain }}"  # Paperless-NGX
   syncthing: "syncthing.{{ primary_domain }}"