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>

README.md

@@ -53,6 +53,7 @@ The **Falcon** is a state-of-the-art containerized starship, powered by Docker's
 | **STASH** | *Universal file management portal* | [stash.pivoine.art](https://stash.pivoine.art) |
 | **LINKS** | *Interstellar bookmark archive* | [links.pivoine.art](https://links.pivoine.art) |
 | **VAULT** | *Encrypted password vault* | [vault.pivoine.art](https://vault.pivoine.art) |
+| **JOPLIN** | *Note-taking server & sync hub* | [joplin.pivoine.art](https://joplin.pivoine.art) |
 | **RESTIC** | *Automated backup vault system* | [restic.pivoine.art](https://restic.pivoine.art) |
 | **PROXY** | *Shield control dashboard* | [proxy.pivoine.art](https://proxy.pivoine.art) |
 | **VPN** | *Cloaking device network* | [vpn.pivoine.art](https://vpn.pivoine.art) |
@@ -72,7 +73,8 @@ The **Falcon** is a state-of-the-art containerized starship, powered by Docker's
 │  ├─ Directus Sector Database                   │
 │  ├─ Umami Analytics Vault                      │
 │  ├─ n8n Workflow Engine Database               │
-│  └─ Linkwarden Bookmark Archive                │
+│  ├─ Linkwarden Bookmark Archive                │
+│  └─ Joplin Note-taking Server Database         │
 ├─────────────────────────────────────────────────┤
 │  ⚡ REDIS CACHE HYPERDRIVE                     │
 │  └─ Warp-speed data acceleration               │
@@ -206,6 +208,7 @@ THE FALCON (falcon_network)
 │  ├─ Filestash Files    [stash.pivoine.art]
 │  ├─ Linkwarden Marks   [links.pivoine.art]
 │  ├─ Vaultwarden Vault  [vault.pivoine.art]
+│  ├─ Joplin Sync Server [joplin.pivoine.art]
 │  ├─ Backrest Backups   [restic.pivoine.art]
 │  └─ WireGuard VPN      [vpn.pivoine.art]
 │
@@ -221,6 +224,7 @@ THE FALCON (falcon_network)
    ├─ linkwarden_data    → Bookmark archives
    ├─ meili_data         → Search index database
    ├─ vaultwarden_data   → Encrypted password vault
+   ├─ joplin_data        → Note-taking server data
    ├─ backrest_data      → Backup system state
    ├─ backrest_config    → Backup configurations
    └─ letsencrypt_data   → Shield certificates