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/config.json

@@ -210,6 +210,22 @@
           "yearly": 3
         }
       }
+    },
+    {
+      "id": "joplin-backup",
+      "repo": "hidrive-backup",
+      "paths": ["/volumes/joplin_data"],
+      "schedule": {
+        "cron": "0 2 * * *"
+      },
+      "retention": {
+        "policyTimeBucketed": {
+          "daily": 7,
+          "weekly": 4,
+          "monthly": 6,
+          "yearly": 2
+        }
+      }
     }
   ]
 }