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/alertmanager.yaml

@@ -1,5 +1,10 @@
 route:
   receiver: default
+  routes:
+    - receiver: "null"
+      matchers:
+        - alertname="Watchdog"
 
 receivers:
   - name: default
+  - name: "null"