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>

_backup/.env.example

@@ -0,0 +1,3 @@
+RESTIC_REPOSITORY=/mnt/hidrive/users/valknar/Backup/stacks
+RESTIC_PASSWORD=change_me
+WEBHOOK_URL=https://n8n.example.com/webhook/change_me

_backup/backup.sh

@@ -18,9 +18,9 @@ log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE"; }
 
 notify() {
   local color="$1" text="$2"
-  curl -sf -o /dev/null -X POST "$MATTERMOST_WEBHOOK" \
+  curl -sf -o /dev/null -X POST "$WEBHOOK_URL" \
     -H 'Content-Type: application/json' \
-    -d "{\"attachments\":[{\"color\":\"$color\",\"text\":\"$text\"}]}"
+    -d "{\"text\":\"$text\",\"color\":\"$color\"}"
 }
 
 # Truncate log on each run
@@ -35,7 +35,7 @@ declare -A DATABASES=(
   [umami_db]="umami:umami"
   [blinko_db]="blinko:blinko"
   [gitea_db]="gitea:gitea"
-  [mattermost_db]="mattermost:mattermost"
+  [n8n_db]="n8n:n8n"
   [immich_db]="immich:immich"
   [coolify_db]="coolify:coolify"
 )