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>

CLAUDE.md

@@ -21,6 +21,7 @@ Root `compose.yaml` uses Docker Compose's `include` directive to orchestrate mul
 - **stash**: Filestash web-based file manager
 - **links**: Linkwarden bookmark manager (PostgreSQL + Meilisearch)
 - **vault**: Vaultwarden password manager (SQLite)
+- **joplin**: Joplin Server note-taking and sync platform (PostgreSQL)
 - **restic**: Backrest backup system with restic backend
 - **sablier**: Dynamic scaling plugin for Traefik
 - **vpn**: WireGuard VPN (wg-easy)
@@ -57,6 +58,7 @@ Services expose themselves via Docker labels:
 - Creates `umami` database for Track analytics
 - Creates `n8n` database for workflow automation
 - Creates `linkwarden` database for Links bookmark manager
+- Creates `joplin` database for Joplin Server
 - Grants privileges to `$DB_USER`
 
 ## Common Commands
@@ -221,6 +223,30 @@ Vaultwarden password manager (Bitwarden-compatible server):
 - Enable 2FA for all accounts
 - Access admin panel at `/admin` (requires `ADMIN_TOKEN` in `.env`)
 
+### Joplin (joplin/compose.yaml)
+Joplin Server note-taking and synchronization platform:
+- **joplin**: Joplin Server app exposed at `joplin.pivoine.art:22300`
+  - Self-hosted sync server for Joplin note-taking clients
+  - End-to-end encryption support for notebooks
+  - Multi-device synchronization (desktop, mobile, CLI)
+  - Markdown-based notes with attachments
+  - PostgreSQL backend for data persistence
+  - Compatible with official Joplin clients (Windows, macOS, Linux, Android, iOS)
+  - Data persisted in PostgreSQL `joplin` database
+
+**Configuration**:
+- **APP_BASE_URL**: `https://joplin.pivoine.art` (required for client synchronization)
+- **APP_PORT**: `22300` (Joplin Server default port)
+- **DB_CLIENT**: `pg` (PostgreSQL database driver)
+- Uses shared core PostgreSQL instance
+
+**Usage**:
+1. Access https://joplin.pivoine.art to create an account
+2. In Joplin desktop/mobile app, go to Settings → Synchronization
+3. Select "Joplin Server" as sync target
+4. Enter server URL: `https://joplin.pivoine.art`
+5. Enter email and password created in step 1
+
 ### Restic (restic/compose.yaml)
 Backrest backup system with restic backend:
 - **backrest**: Backrest web UI exposed at `restic.pivoine.art:9898`
@@ -290,6 +316,10 @@ Backrest backup system with restic backend:
     - Path: `/volumes/vaultwarden_data`
     - Retention: 7 daily, 4 weekly, 12 monthly, 3 yearly
 
+13. **joplin-backup** (2 AM daily)
+    - Path: `/volumes/joplin_data`
+    - Retention: 7 daily, 4 weekly, 6 monthly, 2 yearly
+
 **Volume Mounting**:
 All Docker volumes are mounted read-only to `/volumes/` with prefixed names (e.g., `backup_core_postgres_data`) to avoid naming conflicts with other compose stacks.