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

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "s5p"
-version = "0.1.0"
+version = "0.3.0"
 description = "SOCKS5 proxy with Tor and proxy-chain support"
 requires-python = ">=3.11"
 dependencies = ["pyyaml>=6.0"]