feat: add Mattermost volumes to Restic backup

Added Mattermost volumes to backup configuration:
- mattermost_config: Server configuration and settings
- mattermost_data: User data, posts, files, and attachments
- mattermost_plugins: Installed plugins

These volumes contain critical Mattermost data and should be
backed up regularly to ensure team chat history and configurations
can be restored if needed.

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

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

restic/compose.yaml

@@ -20,6 +20,9 @@ services:
       - backup_directus_uploads:/volumes/directus_uploads:ro
       - backup_directus_bundle:/volumes/directus_bundle:ro
       - backup_awesome_data:/volumes/awesome_data:ro
+      - backup_mattermost_config:/volumes/mattermost_config:ro
+      - backup_mattermost_data:/volumes/mattermost_data:ro
+      - backup_mattermost_plugins:/volumes/mattermost_plugins:ro
       - backup_scrapyd_data:/volumes/scrapyd_data:ro
       - backup_scrapy_code:/volumes/scrapy_code:ro
       - backup_n8n_data:/volumes/n8n_data:ro
@@ -83,6 +86,15 @@ volumes:
   backup_awesome_data:
     name: awesome_data
     external: true
+  backup_mattermost_config:
+    name: mattermost_config
+    external: true
+  backup_mattermost_data:
+    name: mattermost_data
+    external: true
+  backup_mattermost_plugins:
+    name: mattermost_plugins
+    external: true
   backup_scrapyd_data:
     name: scrapy_scrapyd_data
     external: true