Add Apache RAT for release-time license audit

## Summary
Apache RAT (Release Audit Tool) is **required by ASF release policy** to verify that every
file in a source release carries an approved license header. Add RAT and a `.rat-excludes`
(or `rat-config.xml`) exclusion list for files that are legitimately header-free (test
fixtures, generated files, binary assets, etc.).

## Motivation
- ASF release policy mandates a clean RAT run before a release vote
- Apache Airflow maintains ~334 exclusion patterns as a reference
- JDK 21 is already a prerequisite for Solr, so the Maven/RAT toolchain is available

## Proposed changes
1. Add a `rat-config.xml` (or `.rat-excludes` file) listing known exclusions
2. Add a `Makefile` target `make rat` that runs RAT against the source tree
3. Integrate the RAT check into the release checklist / CI release workflow
4. Document the step in `DEVELOPER_GUIDE.md` under a "Release" section

## References
- https://creadur.apache.org/rat/
- ASF release policy: https://www.apache.org/legal/release-policy.html