Bump healthcheck to Node 20; disable pointless cron schedule

healthcheck.js only opens the local committed SQLite DB on the runner
and never contacts the deployed server, so the every-15-min schedule
validated nothing and consumed Actions minutes. Keep workflow_dispatch
for manual runs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/server-healthcheck.yml

@@ -2,8 +2,10 @@ name: Server Healthcheck
 
 on:
   workflow_dispatch:
-  schedule:
-    - cron: '*/15 * * * *' # every 15 minutes
+  # NOTE: scheduled runs disabled. healthcheck.js only opens the local
+  # committed SQLite DB on the runner (it does not reach the deployed
+  # server), so a cron schedule here validated nothing and burned Actions
+  # minutes. Run manually via the Actions tab when needed.
 
 jobs:
   healthcheck:
@@ -13,7 +15,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
       - name: Install dependencies
         run: |
           cd database