add backend setup docs

src/backend.md

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+# W3Registrar Setup Guide
+
+## Prerequisites
+- Redis server (not needed for Docker setup)
+- `config.toml` (copy from `config.example.toml`)
+- Matrix account credentials
+
+## 1. Docker Setup
+
+1. Create configuration:
+```bash
+cp config.example.toml config.toml
+# Edit config.toml with your settings
+# Important: use "redis" as host in [redis] section
+```
+
+2. Run services:
+```bash
+docker compose up -d
+```
+
+## 2. Binary Installation
+
+1. Create service user:
+```bash
+sudo useradd -r -s /bin/false w3r
+sudo mkdir -p /etc/w3registrar
+sudo chown -R w3r:w3r /etc/w3registrar
+```
+
+2. Install binary:
+```bash
+# Download from releases page
+sudo cp w3registrar-* /usr/local/bin/w3registrar
+sudo chown w3r:w3r /usr/local/bin/w3registrar
+sudo chmod 755 /usr/local/bin/w3registrar
+```
+
+3. Configure:
+```bash
+sudo cp config.toml /etc/w3registrar/
+sudo chown w3r:w3r /etc/w3registrar/config.toml
+sudo chmod 600 /etc/w3registrar/config.toml
+```
+
+4. Create systemd service:
+```bash
+sudo tee /etc/systemd/system/w3registrar.service << EOF
+[Unit]
+Description=W3 Registrar Service
+After=network.target redis.service
+
+[Service]
+Type=simple
+User=w3r
+Group=w3r
+ExecStart=/usr/local/bin/w3registrar
+Restart=on-failure
+Environment=RUST_LOG=info
+
+[Install]
+WantedBy=multi-user.target
+EOF
+
+sudo systemctl enable --now w3registrar
+```
+
+## 3. Building from Source
+
+```bash
+# Install dependencies
+sudo apt install -y build-essential pkg-config libssl-dev git
+
+# Build
+git clone https://github.com/your-repo/w3registrar.git
+cd w3registrar
+cargo build --release
+
+# Follow binary installation steps 1-4 above
+```
+
+## Configuration Example
+
+```toml
+[watcher]
+endpoint = "wss://dev.rotko.net/people-rococo/"
+registrar_index = 0
+keystore_path = "./keyfile"
+
+[websocket]
+host = "127.0.0.1"
+port = 8080
+
+[redis]
+host = "redis"  # Use "redis" for Docker, "127.0.0.1" otherwise
+port = 6379
+
+[matrix]
+homeserver = "https://matrix.org"
+username = "regbot"
+password = "your-password"
+security_key = "your-security-key"
+admins = ["@admin:matrix.org"]
+```
+
+## Common Issues
+
+- **Redis Connection**: Check if Redis is running and host configuration
+- **Matrix Login**: Verify credentials and security key
+- **Permissions**: Ensure proper file ownership and permissions under w3r user