docs: add Controller + Compute setup guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/features/compute-controller-setup.md

@@ -0,0 +1,96 @@
+# Controller + Compute Setup
+
+This document describes the minimum configuration required to set up a GNS3 Controller with remote Compute nodes.
+
+## Architecture Overview
+
+- **Compute**: Runs individual nodes (QEMU, Docker, etc.) and provides resource monitoring
+- **Controller**: Manages multiple computes, projects, and provides the REST API
+- **Database**: Controller uses SQLite to store projects, nodes, and compute registration
+
+## Minimum Configuration
+
+### 1. Compute Node Configuration
+
+Create the configuration file at `~/.config/GNS3/3.1/gns3_server.conf`:
+
+```ini
+[Server]
+host = 0.0.0.0
+port = 3080
+compute_username = gns3
+compute_password = gns3
+```
+
+Start the Compute:
+```bash
+gns3server
+```
+
+### 2. Controller Node Configuration
+
+Create the configuration file at `~/.config/GNS3/3.1/gns3_server.conf`:
+
+```ini
+[Server]
+host = 192.168.1.140
+port = 3080
+compute_username = gns3
+compute_password = gns3
+```
+
+Start the Controller:
+```bash
+gns3server
+```
+
+### 3. Register Compute with Controller
+
+Use the API to register a remote compute:
+
+```bash
+POST /v3/computes
+{
+  "protocol": "http",
+  "host": "192.168.1.x",
+  "port": 3080,
+  "user": "gns3",
+  "password": "gns3"
+}
+```
+
+## Important Notes
+
+### Host Configuration
+
+- **Controller `host`**: If set to `0.0.0.0`, it will be changed to `127.0.0.1`, which breaks cross-subnet link creation
+- **Solution**: Always use the actual IP address for Controller's `host` field
+
+### Password Configuration
+
+- If `compute_password` is not set, a random 16-character password is auto-generated on startup
+- The Controller must use the same credentials as the Compute's configuration
+
+### Network Requirements
+
+- Controller and Compute must be on the same LAN for cross-compute links to work
+- UDP tunnel is used for cross-compute links, requiring network connectivity on UDP ports
+
+### Configuration File Location
+
+- Default location: `~/.config/GNS3/3.1/gns3_server.conf`
+- Version `3.0` uses `~/.config/GNS3/3.0/`
+
+## Troubleshooting
+
+### 401 Unauthorized on Compute Connect
+
+1. Check that Compute's `compute_username` and `compute_password` match what was passed to the API
+2. Verify the Compute's configuration file is correctly loaded
+3. Ensure the `[Server]` section is used (not `[Controller]`)
+
+### No Common Subnet Error
+
+1. Verify Controller's `host` is set to its actual IP, not `0.0.0.0`
+2. Ensure both machines are on the same network
+3. Check firewall rules allow UDP communication