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

Makefile

@@ -1,10 +1,17 @@
 APP_NAME := s5p
 
-.PHONY: install test lint clean build up down logs
+.PHONY: install install-service test lint clean build up down logs
 
 install:
 	pip install -e .
 
+install-service:
+	sudo mkdir -p /etc/s5p
+	sudo cp config/s5p.service /etc/systemd/system/s5p.service
+	sudo systemctl daemon-reload
+	@echo "Unit installed. Configure /etc/s5p/s5p.yaml, then:"
+	@echo "  sudo systemctl enable --now s5p"
+
 test:
 	pytest tests/ -v