feat: add Mattermost notifications to Backrest via Shoutrrr hooks

Added global notification hook for all backup operations (backup, prune, check)
using Shoutrrr with Mattermost webhook.

Configuration:
- Hook ID: mattermost-notifications
- Conditions: * (all operations)
- Shoutrrr URL: mattermost://mattermost.pivoine.art/nxsqk4x86jr9bgniheaksem8qy
- Template: Shows operation type, plan/repo ID, status, and errors

Notifications will be sent for:
- All backup operations (17 daily backup plans)
- Weekly prune operations (Sundays at 2 AM)
- Weekly check operations (Sundays at 3 AM)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

restic/config.json

@@ -1,6 +1,16 @@
 {
   "version": 4,
   "instance": "falcon",
+  "hooks": [
+    {
+      "id": "mattermost-notifications",
+      "conditions": ["*"],
+      "actionShoutrrr": {
+        "shoutrrrUrl": "mattermost://mattermost.pivoine.art/nxsqk4x86jr9bgniheaksem8qy",
+        "template": "Backrest notification:\n{{if .Task.Op.OperationBackup}}Backup {{.Plan.Id}} - {{.Status}}{{end}}{{if .Task.Op.OperationPrune}}Prune {{.Repo.Id}} - {{.Status}}{{end}}{{if .Task.Op.OperationCheck}}Check {{.Repo.Id}} - {{.Status}}{{end}}\n{{if .Error}}Error: {{.Error}}{{end}}"
+      }
+    }
+  ],
   "repos": [
     {
       "id": "hidrive-backup",