test: add apply_schedule() unit tests + pycache gitignore

apply_schedule() was the actual behavioral fix in the prior commit but
shipped without tests. Six new pytest cases now cover its five+ state
combinations:

- manual mode with an existing crontab + live crond pid -> crontab
  removed, SIGHUP sent
- manual mode with no crontab and no pid file -> no-op (function
  returns cleanly)
- real schedule with a live pid -> crontab written, SIGHUP sent
- real schedule with a dead pid (ProcessLookupError) -> crontab written,
  fresh crond spawned, new pid file
- real schedule with no pid file (e.g. container started in manual
  mode, user enables a schedule via UI) -> crontab written, fresh
  crond spawned
- real schedule with a garbage pid file (non-integer contents) ->
  ValueError caught, fresh crond spawned

os.kill and subprocess.Popen are mocked so no real signals fire and no
real processes spawn during tests. CRONTAB_FILE / CROND_PID_FILE are
redirected to tmp paths via monkeypatch.

.gitignore: add __pycache__/, *.pyc, and .pytest_cache/ to prevent
future contributors from accidentally committing test artifacts.

.gitignore

@@ -5,6 +5,9 @@
 test/
 downloads/
 config/
+__pycache__/
+*.pyc
+.pytest_cache/
 
 # Editor
 .vscode/