License peo4c under GPL-3.0-or-later and ship third-party notices

- Add the GPL-3.0 LICENSE and SPDX headers to peo4c's own files.
- Add THIRD_PARTY_NOTICES.md listing every bundled component, its
  license and where its license text lives in the image, plus a
  trademark disclaimer; reference it from the README.
- Ship neo4j-browser's LICENSE, NOTICE.txt and LICENSES.txt with a
  source archive of the built commit, as the GPL requires.
- Ship Apache AGE's LICENSE and NOTICE, which the base image lacks.
- Collect license texts for Python packages whose wheels omit them,
  from PyPI sdists or vendored copies in docker/licenses/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker/licenses/README.md

@@ -0,0 +1,11 @@
+# Vendored license texts
+
+License files for Python packages that pgAdmin depends on whose PyPI releases do
+not include one. `docker/collect-python-licenses.py` copies them into the image.
+Directory names are the PyPI project names.
+
+| Package | License | Source of the text |
+| --- | --- | --- |
+| dnspython | ISC | https://github.com/rthalley/dnspython/blob/v2.9.0/LICENSE |
+| Flask-Principal | MIT | https://github.com/mattupstate/flask-principal/blob/0.4.0/LICENSE |
+| ua-parser-builtins | Apache-2.0 | https://github.com/ua-parser/uap-python/blob/master/LICENSE; it bundles data from uap-core: https://github.com/ua-parser/uap-core/blob/master/LICENSE |