docs: add systemd unit files with EnvironmentFile directive

Unit files load env vars from /etc/central/central.env using
EnvironmentFile directive. Includes README with installation
and configuration instructions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

systemd/README.md

@@ -0,0 +1,41 @@
+# Systemd Unit Files
+
+These unit files configure Central services for systemd.
+
+## Installation
+
+```bash
+# Copy unit files
+sudo cp central-supervisor.service /etc/systemd/system/
+sudo cp central-archive.service /etc/systemd/system/
+
+# Reload systemd
+sudo systemctl daemon-reload
+
+# Enable and start services
+sudo systemctl enable --now central-supervisor
+sudo systemctl enable --now central-archive
+```
+
+## Configuration
+
+Both services load environment variables from `/etc/central/central.env`:
+
+```bash
+CENTRAL_DB_DSN=postgresql://central:password@localhost/central
+CENTRAL_NATS_URL=nats://localhost:4222
+CENTRAL_CONFIG_SOURCE=db
+CENTRAL_MASTER_KEY_PATH=/etc/central/master.key
+```
+
+## Service Dependencies
+
+- **central-supervisor**: Requires NATS server
+- **central-archive**: Requires NATS server and PostgreSQL
+
+## Logs
+
+```bash
+journalctl -u central-supervisor -f
+journalctl -u central-archive -f
+```

systemd/central-archive.service

@@ -10,6 +10,7 @@ User=central
 Group=central
 WorkingDirectory=/opt/central
 Environment=HOME=/opt/central
+EnvironmentFile=/etc/central/central.env
 ExecStart=/opt/central/.venv/bin/central-archive
 Restart=on-failure
 RestartSec=5

systemd/central-supervisor.service

@@ -10,6 +10,7 @@ User=central
 Group=central
 WorkingDirectory=/opt/central
 Environment=HOME=/opt/central
+EnvironmentFile=/etc/central/central.env
 ExecStart=/opt/central/.venv/bin/central-supervisor
 Restart=on-failure
 RestartSec=5