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/CHEATSHEET.md

@@ -21,6 +21,18 @@ s5p --tracemalloc                           # memory profile (top 10)
 s5p --tracemalloc 20                        # memory profile (top 20)
 ```
 
+## Systemd
+
+```
+make install-service                            # install unit + reload
+sudo systemctl enable --now s5p                 # enable + start
+sudo systemctl status s5p                       # check status
+sudo systemctl restart s5p                      # restart
+sudo systemctl stop s5p                         # stop
+journalctl -u s5p -f                            # follow logs
+journalctl -u s5p --since "5 min ago"           # recent logs
+```
+
 ## Container
 
 ```