feat: v0.3.0 stabilization -- systemd, tests, API docs

- Bump version 0.1.0 -> 0.3.0
- Add systemd service unit (config/s5p.service) and install-service
  Makefile target
- Add CLI argument parsing tests (tests/test_cli.py, 27 tests)
- Expand protocol tests with SOCKS5/4/HTTP handshake, error, and auth
  coverage (tests/test_proto.py, 30 tests)
- Add full API reference to docs/USAGE.md with response schemas for
  all GET/POST endpoints
- Update INSTALL.md, CHEATSHEET.md with systemd section
- Update ROADMAP.md, TASKS.md for v0.3.0

docs/INSTALL.md

@@ -42,6 +42,37 @@ The Alpine-based image (~59MB) contains Python, PyYAML, and baked-in
 source. Config is mounted at runtime. The compose.yaml volume mount
 overrides source for local dev.
 
+## Systemd Service
+
+Install s5p as a systemd service for automatic startup and restart.
+
+```bash
+# Install the binary
+cd ~/git/s5p
+source .venv/bin/activate
+pip install -e .
+
+# Copy config
+sudo mkdir -p /etc/s5p
+sudo cp config/example.yaml /etc/s5p/s5p.yaml
+sudo nano /etc/s5p/s5p.yaml                    # edit with your settings
+
+# Install the unit (copies service file + daemon-reload)
+make install-service
+
+# Enable and start
+sudo systemctl enable --now s5p
+
+# Check status
+sudo systemctl status s5p
+journalctl -u s5p -f
+```
+
+The service unit expects:
+- Binary at `/usr/local/bin/s5p`
+- Config at `/etc/s5p/s5p.yaml`
+- Restarts on failure with 5-second delay
+
 ## Install Tor (optional)
 
 ```bash