Add liccheck to gate CI on dependency license compliance

## Summary
Add [`liccheck`](https://github.com/dhatim/python-liccheck) with an authorised-license
allowlist in `setup.cfg` so that CI fails if any (transitive) Python dependency carries an
unapproved license (e.g. GPL, LGPL, or ASF Category X licenses).

## Motivation
- Apache Superset uses `liccheck` to automatically flag problematic transitive deps
- Known issue: `certifi` is MPL-2.0 (ASF Category B — allowed with attribution, not
bundled); liccheck will surface this and similar cases
- Avoids manual license audits when adding or upgrading dependencies

## Proposed changes
1. Add `[liccheck]` section to `setup.cfg` (or a `liccheck.ini`) with:
- `authorized_licenses` allowlist (Apache-2.0, MIT, BSD-*, ISC, Python-2.0, …)
- `unauthorized_licenses` blocklist (GPL, LGPL, AGPL, …)
2. Add `liccheck` to dev/CI dependencies
3. Add `make liccheck` target
4. Integrate into CI pipeline (fail on unapproved licenses)
5. Document in `DEVELOPER_GUIDE.md`

## References
- https://github.com/dhatim/python-liccheck