fix(cluster): support external master URL

- add server.external_url / BACKUPX_SERVER_EXTERNAL_URL for Agent install URL generation
- pass the configured external Master URL into install script and compose rendering
- document cluster deployment requirements for Docker, bare-metal, and multi-node setups

Fixes #55

docs-site/docs/deployment/bare-metal.md

@@ -25,6 +25,19 @@ The installer performs these steps automatically:
 4. Installs `backupx.service` (systemd), enabled at boot
 5. (Optional) installs an Nginx site file — see [Nginx Reverse Proxy](./nginx)
 
+For multi-node clusters, edit `/etc/backupx/config.yaml` after installation and set the Master URL that remote Agents can reach:
+
+```yaml
+server:
+  external_url: "https://backup.example.com"
+```
+
+Restart BackupX after changing it:
+
+```bash
+sudo systemctl restart backupx
+```
+
 ## From source
 
 ```bash

docs-site/docs/deployment/configuration.md

@@ -15,13 +15,14 @@ server:
   host: "0.0.0.0"             # BACKUPX_SERVER_HOST
   port: 8340                  # BACKUPX_SERVER_PORT
   mode: "release"             # release | debug
+  external_url: ""            # BACKUPX_SERVER_EXTERNAL_URL — public Master URL for Agent install scripts
 
 database:
   path: "./data/backupx.db"   # BACKUPX_DATABASE_PATH — embedded SQLite
 
 security:
   jwt_secret: ""              # BACKUPX_SECURITY_JWT_SECRET — auto-generated if empty
-  jwt_expires_in: "24h"
+  jwt_expire: "24h"           # BACKUPX_SECURITY_JWT_EXPIRE
   encryption_key: ""          # AES-256-GCM key for storage config encryption
 
 backup:
@@ -46,7 +47,20 @@ The environment wins when both file and env are set. All dot-paths become unders
 | Config key | Env variable |
 |------------|--------------|
 | `server.port` | `BACKUPX_SERVER_PORT` |
+| `server.external_url` | `BACKUPX_SERVER_EXTERNAL_URL` |
+| `security.jwt_expire` | `BACKUPX_SECURITY_JWT_EXPIRE` |
 | `log.level` | `BACKUPX_LOG_LEVEL` |
 | `backup.max_concurrent` | `BACKUPX_BACKUP_MAX_CONCURRENT` |
 | `backup.temp_dir` | `BACKUPX_BACKUP_TEMP_DIR` |
 | `backup.bandwidth_limit` | `BACKUPX_BACKUP_BANDWIDTH_LIMIT` |
+
+## Master external URL
+
+Set `server.external_url` when BackupX is behind Docker, Nginx, a load balancer, or any reverse proxy whose internal Host is not reachable by remote Agents:
+
+```yaml
+server:
+  external_url: "https://backup.example.com"
+```
+
+This value is used when BackupX renders one-click Agent install scripts and docker-compose snippets. It must be reachable from every Agent host. Leave it empty only when `X-Forwarded-Proto` / `X-Forwarded-Host` are reliable and point to the same URL that Agents can access.

docs-site/docs/deployment/docker.md

@@ -25,6 +25,8 @@ services:
       - /etc/nginx:/mnt/nginx-conf:ro
     environment:
       - TZ=Asia/Shanghai
+      # Required when remote Agents must connect through a public or routed URL:
+      # - BACKUPX_SERVER_EXTERNAL_URL=https://backup.example.com
       - BACKUPX_LOG_LEVEL=info
       - BACKUPX_BACKUP_MAX_CONCURRENT=2
 
@@ -42,6 +44,17 @@ docker compose up -d
 
 To back up files from the host, mount them into the container. When creating a file-type task in the web UI, point the source path at the mount location (e.g. `/mnt/www`). Make sure the directory is visible inside the container.
 
+## Multi-node clusters
+
+When deploying Agents on other machines, set `BACKUPX_SERVER_EXTERNAL_URL` on the Master container to the URL that those Agents can reach:
+
+```yaml
+environment:
+  - BACKUPX_SERVER_EXTERNAL_URL=https://backup.example.com
+```
+
+Use an HTTPS URL if Agents cross untrusted networks. The generated one-click install scripts and docker-compose snippets use this value as `BACKUPX_AGENT_MASTER`.
+
 ## Environment variables
 
 All configuration keys can be overridden with the `BACKUPX_` prefix:

docs-site/docs/features/multi-node.md

@@ -28,6 +28,19 @@ BackupX supports Master-Agent mode: backup tasks can be routed to specific nodes
 
 ## Walkthrough
 
+### 0. Set the Master URL for production clusters
+
+Before generating Agent install commands, make sure the Master URL shown to Agents is stable and reachable from every target host.
+
+If BackupX runs behind Docker, Nginx, a load balancer, or an outer reverse proxy, configure `server.external_url` or `BACKUPX_SERVER_EXTERNAL_URL` on the Master:
+
+```yaml title="config.yaml"
+server:
+  external_url: "https://backup.example.com"
+```
+
+This URL is baked into systemd units, foreground commands, and docker-compose snippets. If it is wrong, Agents will install successfully but stay offline because they keep polling an internal or browser-only address.
+
 ### 1. Open the install wizard
 
 In the Web Console → **Node Management** → **Add Node**. You'll see a three-step wizard.