feat: add Restic backup stack with Backrest UI

Added comprehensive backup solution to The Falcon infrastructure:

- **Restic Stack** (restic.pivoine.art):
  - Backrest web UI for managing restic backups
  - Automated scheduled backups with retention policies
  - Real-time backup status and monitoring
  - Restore capabilities via web interface

- **Backup Configuration**:
  - Target: /mnt/hidrive/users/valknar/Backup
  - Backs up all critical Docker volumes read-only:
    - PostgreSQL, Redis, Directus (uploads/bundle)
    - Awesome, Gotify, Scrapy (data/code)
    - n8n workflows, Filestash state
    - Linkwarden bookmarks/search index
    - Let's Encrypt SSL certificates

- **Infrastructure Updates**:
  - Added RESTIC_* environment variables to arty.yml
  - Updated compose.yaml to include restic stack
  - Updated README.md and CLAUDE.md documentation
  - Configured Traefik routing with SSL

All volumes mounted read-only to backup container for safety.
Backrest data persisted across: data, config, cache, tmp volumes.

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

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

compose.yaml

@@ -8,6 +8,7 @@ include:
   - n8n/compose.yaml
   - stash/compose.yaml
   - links/compose.yaml
+  - restic/compose.yaml
   - umami/compose.yaml
   - sablier/compose.yaml
   - proxy/compose.yaml