tests: retarget /clear_pr_pls auth tests after the musician/radio split

test_musician_auth.py still probed /clear_pr_pls on the musician, but that
endpoint moved to betoniarka during the split - the musician now 404s it, so
all three tests failed 404 != 401/200. This was pre-existing debt, unrelated to
the AI/share/bridge work; it just kept the integration job red.

Split the coverage to match the current architecture:
* test_musician_auth.py exercises the same auth contract (no key -> 401, key ->
  200, key unset -> open) against /get_share_list, an authenticated endpoint the
  musician still serves, with a valid body so the permitted case is a clean 200
  rather than a 400;
* new test_betoniarka_auth.py covers /clear_pr_pls where it now lives, pointing
  PRIORITY_PLAYLIST_PATH at a tmp file so the authorised case can truncate it,
  and checks /ping stays open;
* conftest.py adds conjurer_betoniarka to sys.path so the service imports.

Verified in a clean venv (pytest flask waitress requests), matching the CI
integration job: 13 passed, up from 3 failed / 6 passed. Unit suite unaffected
(23 passed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

conftest.py

@@ -13,6 +13,7 @@ for _path in (
     _ROOT,
     os.path.join(_ROOT, "conjurer_musician"),
     os.path.join(_ROOT, "conjurer_librarian"),
+    os.path.join(_ROOT, "conjurer_betoniarka"),
 ):
     if _path not in sys.path:
         sys.path.insert(0, _path)