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>

core/postgres/init/01-init-databases.sh

@@ -25,16 +25,21 @@ psql -v ON_ERROR_STOP=1 --username "$POSTGRES_USER" --dbname "$POSTGRES_DB" <<-E
     SELECT 'CREATE DATABASE linkwarden'
     WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'linkwarden')\gexec
 
+    -- Joplin note-taking server database
+    SELECT 'CREATE DATABASE joplin'
+    WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'joplin')\gexec
+
     -- Grant privileges to all databases
     GRANT ALL PRIVILEGES ON DATABASE directus TO $POSTGRES_USER;
     GRANT ALL PRIVILEGES ON DATABASE umami TO $POSTGRES_USER;
     GRANT ALL PRIVILEGES ON DATABASE n8n TO $POSTGRES_USER;
     GRANT ALL PRIVILEGES ON DATABASE linkwarden TO $POSTGRES_USER;
+    GRANT ALL PRIVILEGES ON DATABASE joplin TO $POSTGRES_USER;
 
     -- Log success
     SELECT 'Compose databases initialized:' AS status;
     SELECT datname FROM pg_database
-    WHERE datname IN ('directus', 'umami', 'n8n', 'linkwarden')
+    WHERE datname IN ('directus', 'umami', 'n8n', 'linkwarden', 'joplin')
     ORDER BY datname;
 EOSQL
 
@@ -47,4 +52,5 @@ echo "  • directus     - Sexy application database"
 echo "  • umami        - Tracking database"
 echo "  • n8n          - Workflow automation database"
 echo "  • linkwarden   - Bookmark manager database"
+echo "  • joplin       - Note-taking server database"
 echo ""