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

.gitea/workflows/build.yml

@@ -14,10 +14,11 @@ name: Build
 # workflow artifacts (actions/upload-artifact, below) -- it does not create a
 # Gitea Release, push a tag-triggered publish, or otherwise distribute
 # binaries publicly, and it must not start doing so without explicit owner
-# sign-off on the WebRTC/OpenH264 attribution and GPLv2/Apache-2.0
-# license-compatibility questions tracked in third_party/livekit/README.md
-# and the README's top-level Status section. If a real release/publish step
-# is ever added here, it must carry that same gate.
+# sign-off on the WebRTC/OpenH264 attribution question tracked in
+# third_party/livekit/README.md and the README's top-level Status section.
+# (The separate GPLv2/Apache-2.0 license-compatibility question is resolved:
+# this project's own code is Apache-2.0.) If a real release/publish step is
+# ever added here, it must carry that same gate.
 
 on:
   push: