fix(alertmanager): quote null receiver and wire telegram template

Unquoted null parses as YAML null and breaks routing. Add shared telegram message template.

prometheus-stack/k8s/alertmanager-telegram.tmpl

@@ -0,0 +1,44 @@
+{{- define "telegram.forust.message" -}}
+{{- $statusEmoji := "🚨" -}}
+{{- if ne .Status "firing" -}}{{- $statusEmoji = "✅" -}}{{- end -}}
+{{- $count := len .Alerts.Firing -}}
+{{- if eq .Status "resolved" -}}{{- $count = len .Alerts.Resolved -}}{{- end -}}
+{{ $statusEmoji }} <b>{{ .Status | toUpper }} ({{ $count }})</b>
+{{- range .Alerts }}
+
+<b>{{ .Labels.alertname }}</b>
+{{- $sev := .Labels.severity }}
+{{- if eq $sev "critical" }} 🔥 critical
+{{- else if eq $sev "warning" }} ⚠️ warning
+{{- else if eq $sev "info" }} ℹ️ info
+{{- else if $sev }} • {{ $sev }}
+{{- end }}
+{{- if .Annotations.description }}
+
+<i>{{ .Annotations.description }}</i>
+{{- end }}
+{{- if .Labels.namespace }}
+📦 Namespace: <code>{{ .Labels.namespace }}</code>
+{{- end }}
+{{- if .Labels.pod }}
+📦 Pod: <code>{{ .Labels.pod }}</code>
+{{- end }}
+{{- if .Labels.container }}
+🐳 Container: <code>{{ .Labels.container }}</code>
+{{- end }}
+{{- if .Labels.node }}
+🖥 Node: <code>{{ .Labels.node }}</code>
+{{- end }}
+{{- if .Labels.instance }}
+🖥 Instance: <code>{{ .Labels.instance }}</code>
+{{- end }}
+{{- if .Labels.job }}
+🔧 Job: <code>{{ .Labels.job }}</code>
+{{- end }}
+{{- if eq .Status "firing" }}
+🕐 Since: <code>{{ .StartsAt | date "2006-01-02 15:04 MST" }}</code>
+{{- else }}
+🕐 Resolved: <code>{{ .EndsAt | date "2006-01-02 15:04 MST" }}</code>
+{{- end }}
+{{- end }}
+{{- end }}