license: relicense first-party code from GPL-2.0-or-later to Apache-2.0

Owner sign-off: replace root LICENSE with Apache License 2.0, add a root
NOTICE file, and swap the GPL-2.0 boilerplate header in every first-party
core/ and obs-adapter/ source file for a short Apache-2.0 notice.

This resolves review finding C2 (GPLv2 top-level LICENSE vs. the vendored
Apache-2.0 LiveKit SDK is a license-compatibility violation): the whole
repo is now Apache-2.0, matching LiveKit, so there's no GPL/Apache clash
left. Updated the README Status gate and the CI workflow comment to reflect
that C2 is resolved, while leaving the C1 WebRTC/OpenH264 patent/royalty
gate untouched -- that question is still open and still blocks release.

third_party/ stays under its own upstream licenses; only this project's own
code changed hands. All 6 CTest suites still pass after the header swap.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

core/include/stplugin/session.h

@@ -2,18 +2,11 @@
 streamer-tools OBS Camera Plugin - LiveKit session wrapper
 Copyright (C) 2026 CyberCoveLLC <jknapp85@gmail.com>
 
-This program is free software; you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation; either version 2 of the License, or
-(at your option) any later version.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
 
-This program is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-GNU General Public License for more details.
-
-You should have received a copy of the GNU General Public License along
-with this program. If not, see <https://www.gnu.org/licenses/>
+    http://www.apache.org/licenses/LICENSE-2.0
 */
 
 #pragma once