feat(argocd): recover Telegram notifications (failed/degraded) and prune for polymarket-bot

- Notifications via generic webhook to the Telegram Bot API: the engine's
  native telegram service only supports channels and negative group IDs,
  so a positive private chat_id never worked ("chat not found").
- Only on-sync-failed and on-health-degraded triggers; on-sync-succeeded
  dropped (CI already reports successful deploys).
- Subscribe annotations on polymarket-bot, researchowl and n8n.
- prune: true on polymarket-bot, guarded by Prune=false on the postgres
  PVC so removing the manifest from git can never destroy the data.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

argocd/application-n8n.yaml

@@ -3,6 +3,9 @@ kind: Application
 metadata:
   name: n8n
   namespace: argocd
+  annotations:
+    notifications.argoproj.io/subscribe.on-sync-failed.telegram: "5138407666"
+    notifications.argoproj.io/subscribe.on-health-degraded.telegram: "5138407666"
 spec:
   destination:
     namespace: n8n