feat: add Joplin Server stack for note-taking and synchronization

Added Joplin Server deployment at joplin.pivoine.art:

**Joplin stack** (joplin.pivoine.art):
- joplin: Note-taking server with multi-device sync
  - PostgreSQL backend for data persistence
  - End-to-end encryption support
  - Compatible with official Joplin clients (desktop, mobile, CLI)
  - Markdown-based notes with attachments

Infrastructure updates:
- Added joplin database to PostgreSQL init script
- Updated compose.yaml to include joplin stack
- Added JOPLIN_* environment variables to arty.yml
- Added joplin-backup plan to restic (13th backup plan)
- Updated restic/compose.yaml with joplin_data volume mount
- Updated README.md and CLAUDE.md documentation

All services integrated with Traefik for SSL termination and include
Watchtower auto-update labels. Daily backups scheduled for 2 AM with
7 daily, 4 weekly, 6 monthly, and 2 yearly retention.

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

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

restic/compose.yaml

@@ -29,6 +29,7 @@ services:
       - backup_linkwarden_meili_data:/volumes/linkwarden_meili_data:ro
       - backup_letsencrypt_data:/volumes/letsencrypt_data:ro
       - backup_vaultwarden_data:/volumes/vaultwarden_data:ro
+      - backup_joplin_data:/volumes/joplin_data:ro
 
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
@@ -108,6 +109,9 @@ volumes:
   backup_vaultwarden_data:
     name: vault_data
     external: true
+  backup_joplin_data:
+    name: joplin_data
+    external: true
 
 networks:
   compose_network: