feat: replace mattermost with n8n, route notifications to telegram

- Add n8n stack (n8n.pivoine.art) with postgres, traefik, watchtower
- Remove mattermost stack entirely
- Update backup.sh: swap mattermost_db → n8n_db, rename
  MATTERMOST_WEBHOOK → WEBHOOK_URL, simplify notify() payload
- Update watchtower: mattermost:// → generic+https:// for n8n webhook
- Add _backup/.env.example
- Update README

Webhook URLs in _backup/.env and watchtower/.env contain
FILL_AFTER_N8N_SETUP — update once n8n workflow is created.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

n8n/.env.example

@@ -0,0 +1,3 @@
+TRAEFIK_HOST=n8n.example.com
+NETWORK_NAME=falcon_network
+N8N_ENCRYPTION_KEY=change_me