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

third_party/livekit/README.md

@@ -8,7 +8,8 @@ module — so the SDK's licence and notice files ship with it.
 `LICENSE` and `NOTICE` here are copied verbatim from the pinned release tag
 `v1.10.1` (Apache License 2.0). They are staged into `build/package/licenses/`
 by `obs-adapter/CMakeLists.txt` on every build, alongside this plugin's own
-GPL-2.0 `LICENSE`.
+Apache-2.0 `LICENSE` (this project's own first-party code was relicensed from
+GPL-2.0 to Apache-2.0 to match).
 
 ## A correction to the design doc