Add transmission role for torrent-based ZIM downloads

- Add new transmission ansible role using homebrew + brew services
- Configure transmission to download to ~/transmission with localhost-only RPC
- Modify kiwix role to use transmission for downloading ZIM archives via BitTorrent
- Add role dependency so running --tags kiwix auto-runs transmission
- Keep fallback to direct HTTP download when kiwix_use_transmission: false
- Symlink completed downloads from transmission dir to kiwix-tools dir

This reduces load on kiwix.org servers and allows downloads to continue
in the background without blocking ansible runs.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ansible/playbooks/indri.yml

@@ -6,6 +6,8 @@
       tags: prometheus
     - role: grafana
       tags: grafana
+    - role: transmission
+      tags: transmission
     - role: kiwix
       tags: kiwix
     - role: borgmatic