chore: resolve the conflicting license declarations (Apache-2.0 source / GPL-3.0 packages)

[lqh-watch]

What

The repository declared three different licenses for itself, and the release packages shipped with no license text at all.

Where	Before	After
Root LICENSE	GPL-2.0	Apache-2.0 (source)
13 files under src/common, src/component, src/transcoder	LGPL-2.1	Apache-2.0
11 files with no header at all	—	Apache-2.0
Release packages (tar.gz / zip / dmg / layer)	no license text	GPL-3.0 + THIRD_PARTY_NOTICES.md

Why

5f28b6f changed the root LICENSE to GPL-2.0, but that commit touched only that one file — the README, every file header and the developer docs still said Apache-2.0. That
state is also the one option that cannot legally cover this project:

• The packages link against a GPL-enabled FFmpeg — libx264/libx265 from BtbN's *-gpl-shared build, Homebrew ffmpeg@5 on macOS — so copyleft covers the combined
  binary.
• Apache-2.0 is incompatible with GPLv2, and the Apache-2.0 files include work from outside contributors (no CLA/DCO has ever been in place, and the root LICENSE was
  Apache-2.0 when they contributed), so GPL-2.0 cannot absorb them either.
• Qt's open-source options are LGPLv3/GPLv2, and LGPLv3 can only be upgraded to GPLv3, not downgraded to GPLv2.

All three constraints point the same way for the binaries — GPL-3.0 — while the source itself can stay Apache-2.0.

How

Source (0612a78)

1. Restore the root LICENSE to the Apache-2.0 text used before 5f28b6f
2. Replace the LGPL-2.1 header of 13 files with the standard Apache-2.0 header
3. Add the missing header to main.cpp, transcoder_test.cpp, enhance_module.py and the 8 shell scripts under tool/ and src/resources
4. Delete src/common/LICENSE and src/transcoder/LICENSE (the LGPL-2.1 texts)
5. Update README, CLAUDE.md and .augment-guidelines to one consistent story
6. Add .github/pre-commit/check_license_header.py so a new source file cannot land without a header

Packages (be3a35e)

1. Add licenses/GPL-3.0.txt and THIRD_PARTY_NOTICES.md (bundled components, their licenses, and where to obtain the corresponding source)
2. Bundle both plus LICENSE into the Linux tar.gz, the Windows zip, the macOS app (Contents/Resources) and the Linglong layer (share/licenses)
3. Run the macOS step before the bundle is re-signed, so the added files are covered by the signature; the Linglong step drops the copy that arrives with the binary artifact
   to avoid a duplicate under bin/
4. State the source/binary split in README.md / README_ZH.md
5. Document the LGPL vs GPL FFmpeg trade-off in doc/CompileGuide.md
6. Fix the repository URL in the DMG install instructions (it pointed at TSGU-OSC/OpenConverter, a different repository), and add a license note there for end users

Notes

• The LGPL-2.1 headers came from the belief that calling the FFmpeg API makes a file a derivative work of FFmpeg. It does not — those files are the project's own code.
• A binary you build yourself against an LGPL FFmpeg build contains no GPL code and stays under the Apache-2.0 terms.
• This changes the terms of future packages only; versions already released keep the terms they were published with.

Testing

• pre-commit run --files ... — all 11 hooks pass (including the new license-header hook)
• bash -n on every shell script; YAML parse check on all workflows
• Simulated the four packaging paths in a temp tree and verified where the license files land, plus that a re-run does not nest licenses/licenses

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud