Relicense under GPL-3.0-or-later

Replace MIT with the full GNU GPLv3 text, update license metadata in
pyproject.toml (+ trove classifiers) and PKGBUILD, and add
SPDX-License-Identifier headers to all Python modules and shell scripts.

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

enodia_sentinel/detectors/__init__.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
 """Detector registry.
 
 Each detector is a ``Detector`` with a ``name`` and a ``detect(state, cfg)``