refactor(etherpad): address review feedback

- Replace manual tag lookup + git clone/checkout with
  fetch_and_deploy_gh_release (tarball mode) for both install and update
  paths, matching repo convention (see ct/clickhouse.sh).
- Drop git, curl, ca-certificates from apt deps — base image ships them
  and the helper no longer needs git.
- Remove the id -u etherpad guard; fresh LXC will not have the user.
- Bump Debian from 12 to 13 (trixie).
- Switch logo to selfh.st/icons webp.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

json/etherpad.json

@@ -11,7 +11,7 @@
   "interface_port": 9001,
   "documentation": "https://etherpad.org/doc",
   "website": "https://etherpad.org",
-  "logo": "https://raw.githubusercontent.com/ether/etherpad-lite/develop/src/static/favicon.ico",
+  "logo": "https://cdn.jsdelivr.net/gh/selfhst/icons@main/webp/etherpad.webp",
   "description": "Etherpad is a highly customizable real-time collaborative document editor. It lets multiple people edit the same document simultaneously in the browser, with live changes, per-user colors, chat, and a rich plugin ecosystem.",
   "install_methods": [
     {
@@ -23,7 +23,7 @@
         "ram": 2048,
         "hdd": 8,
         "os": "Debian",
-        "version": "12"
+        "version": "13"
       }
     }
   ],