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>

arty.yml

@@ -100,9 +100,16 @@ envs:
     VAULT_IMAGE: vaultwarden/server:latest
     VAULT_TRAEFIK_HOST: vault.pivoine.art
     VAULT_WEBSOCKET_ENABLED: true
-    VAULT_SIGNUPS_ALLOWED: false
+    VAULT_SIGNUPS_ALLOWED: true
     VAULT_INVITATIONS_ALLOWED: true
     VAULT_SHOW_PASSWORD_HINT: false
+    # Joplin
+    JOPLIN_TRAEFIK_ENABLED: true
+    JOPLIN_COMPOSE_PROJECT_NAME: joplin
+    JOPLIN_IMAGE: joplin/server:latest
+    JOPLIN_TRAEFIK_HOST: joplin.pivoine.art
+    JOPLIN_APP_PORT: 22300
+    JOPLIN_DB_NAME: joplin
     # Proxy
     PROXY_COMPOSE_PROJECT_NAME: proxy
     PROXY_DOCKER_IMAGE: traefik:latest