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

config/s5p.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=s5p SOCKS5 proxy
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/s5p -c /etc/s5p/s5p.yaml
+Restart=on-failure
+RestartSec=5
+KillSignal=SIGTERM
+TimeoutStopSec=10
+
+[Install]
+WantedBy=multi-user.target