Generate alertmanager.yml so no-webhook is a valid state, not a broken one

Deploying before the webhook exists is deliberate, so it must not depend on Alertmanager tolerating an empty
webhook_url_file — which I could not verify. configure.py now generates the config and omits the Discord
notifier entirely when ALERT_WEBHOOK_URL is unset: the receiver exists, drops notifications, and alerts still
evaluate, group and show at /alerts. Setting the variable and re-applying turns delivery on. The URL stays in
/etc/vanguard/alert-webhook rather than the config, because that file can be 0640 root:65534 for the
container's user while the repo copy cannot.

alertmanager.example.yml is the reference copy and is gitignored in generated form, like the other configs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

check.sh

@@ -13,7 +13,7 @@ for f in configure.py bin/dumpd.py bin/vgstrip.py; do
   python3 -m py_compile "$f" && say "$f" ok || { say "$f" "PYTHON SYNTAX"; fail=1; }
 done
 rm -rf __pycache__ bin/__pycache__
-python3 -c "import yaml,sys; [yaml.safe_load(open(f)) for f in sys.argv[1:]]" docker-compose.yml mediamtx.example.yml observability/vector.yaml observability/scrape.yml observability/alerts-metrics.yml observability/alerts-logs.yml observability/alertmanager.yml && say "yaml" ok || { say "yaml" "INVALID"; fail=1; }
+python3 -c "import yaml,sys; [yaml.safe_load(open(f)) for f in sys.argv[1:]]" docker-compose.yml mediamtx.example.yml observability/vector.yaml observability/scrape.yml observability/alerts-metrics.yml observability/alerts-logs.yml observability/alertmanager.example.yml && say "yaml" ok || { say "yaml" "INVALID"; fail=1; }
 node -e "
 const fs=require('fs');
 for (const f of ['dock/index.html','dock/clock/index.html','clips/index.html','about/index.html']) { const html=fs.readFileSync(f,'utf8'); for (const m of html.matchAll(/<script(?:[^>]*)>([\s\S]*?)<\/script>/g)) if (m[1].trim()) new Function(m[1]); }