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/compose.yml

@@ -0,0 +1,59 @@
+services:
+  n8n:
+    image: n8nio/n8n:latest
+    container_name: n8n
+    environment:
+      TZ: ${TIMEZONE:-Europe/Amsterdam}
+      N8N_HOST: ${TRAEFIK_HOST}
+      N8N_PORT: 5678
+      N8N_PROTOCOL: https
+      WEBHOOK_URL: https://${TRAEFIK_HOST}/
+      N8N_ENCRYPTION_KEY: ${N8N_ENCRYPTION_KEY}
+      DB_TYPE: postgresdb
+      DB_POSTGRESDB_HOST: n8n_db
+      DB_POSTGRESDB_PORT: 5432
+      DB_POSTGRESDB_DATABASE: n8n
+      DB_POSTGRESDB_USER: n8n
+      DB_POSTGRESDB_PASSWORD: n8n
+    volumes:
+      - ../.data/n8n/data:/home/node/.n8n
+    depends_on:
+      db:
+        condition: service_healthy
+    restart: always
+    labels:
+      - "traefik.enable=true"
+      - "traefik.http.middlewares.n8n-redirect-web-secure.redirectscheme.scheme=https"
+      - "traefik.http.routers.n8n-web.middlewares=n8n-redirect-web-secure"
+      - "traefik.http.routers.n8n-web.rule=Host(`${TRAEFIK_HOST}`)"
+      - "traefik.http.routers.n8n-web.entrypoints=web"
+      - "traefik.http.routers.n8n-web-secure.rule=Host(`${TRAEFIK_HOST}`)"
+      - "traefik.http.routers.n8n-web-secure.tls.certresolver=resolver"
+      - "traefik.http.routers.n8n-web-secure.entrypoints=web-secure"
+      - "traefik.http.routers.n8n-web-secure.middlewares=security-headers@file,no-index@file"
+      - "traefik.http.services.n8n-web-secure.loadbalancer.server.port=5678"
+      - "traefik.docker.network=${NETWORK_NAME}"
+      - "com.centurylinklabs.watchtower.enable=true"
+    networks:
+      - compose_network
+  db:
+    image: postgres:16-alpine
+    container_name: n8n_db
+    environment:
+      POSTGRES_DB: n8n
+      POSTGRES_USER: n8n
+      POSTGRES_PASSWORD: n8n
+    volumes:
+      - ../.data/n8n/db:/var/lib/postgresql/data
+    restart: always
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+    networks:
+      - compose_network
+networks:
+  compose_network:
+    name: ${NETWORK_NAME}
+    external: true