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>

README.md

@@ -14,7 +14,7 @@ Each stack is independently deployable with its own `compose.yml` and `.env`. Al
 | `umami` | Web analytics | umami, db |
 | `immich` | Photo & video management | immich, ml, redis, db |
 | `blinko` | AI-powered personal notes | blinko, db |
-| `mattermost` | Team chat | mattermost, db |
+| `n8n` | Workflow automation & notification relay | n8n, db |
 | `gitea` | Git hosting + CI runner | gitea, runner, db |
 | `coolify` | Deployment platform | coolify, realtime, redis, db |
 | `vaultwarden` | Password manager | vaultwarden |
@@ -44,7 +44,7 @@ All stacks share the external `falcon_network` Docker network for inter-service
 
 ## Backup
 
-The `_backup` stack runs a daily restic backup at 3:00 AM. It dumps all Postgres databases, then backs up the entire `.data/` directory to HiDrive. Retention: 7 daily, 4 weekly, 6 monthly snapshots. Notifications go to Mattermost.
+The `_backup` stack runs a daily restic backup at 3:00 AM. It dumps all Postgres databases, then backs up the entire `.data/` directory to HiDrive. Retention: 7 daily, 4 weekly, 6 monthly snapshots. Notifications go to Telegram via n8n.
 
 ```bash
 # Deploy backup stack