Add a license for the repository and the test books

The repository has no LICENSE file and the README says nothing about licensing, so it isn't clear
whether the test books may be redistributed or bundled into other test suites. That matters to
tools that want to run the epubtest.org books in CI (validators, converters, reading-system test
harnesses). Without a license, the only safe option is to download the books at test time; they
can't be committed as fixtures or shipped with a test suite.

The package documents don't settle it either, because they disagree:

| Book | License or rights metadata in `package.opf` |
|---|---|
| `epub30-test-0301` Basic Functionality | none |
| `epub30-test-0302` Non-Visual Reading | none |
| `epub30-test-0303` Visual Adjustments | none |
| `epub30-test-0304` Read Aloud | none |
| `epub30-test-0320` Media Overlays | `dcterms:license` CC BY 4.0; `dc:rights` "Copyright © 2015 DAISY Consortium" |
| `epub30-test-0330` Math (longer book) | none |
| `epub30-test-0340` Extended Descriptions (longer book) | `dc:rights` CC BY-NC-SA |
| `epub30-test-0350` Extended Descriptions (basic) | `dc:rights` CC BY-NC-SA |
| `epub30-test-0360` Math (basic) | none |
| `epub30-test-0370` | none |
| `epub30-test-exp-01` Reading History Navigation | none |

Would you consider:

1. adding a LICENSE file that states the terms for the book content (text, images, audio) and,
if they differ, for the build scripts;
2. making each book's `dcterms:license` / `dc:rights` match it, since the books also travel on
their own (the releases page, the OPDS feed, epubtest.org)?

If the non-commercial terms on 0340 and 0350 are intended, stating that in the LICENSE would
still help. For comparison, the IDPF EPUB test suite (IDPF/epub-testsuite) is CC BY 4.0, which
lets its books be used in open-source and commercial test tooling alike.

Thank you for these books; they are the most thorough accessibility test content we have found.

Review the README, the proposed LICENSE file, and each book's package.opf metadata, especially the entries for epub30-test-0320, epub30-test-0340, and epub30-test-0350. Confirm the intended terms with maintainers, then ensure the repository documentation and every book's dcterms:license or dc:rights metadata consistently state them for redistribution through releases and the OPDS feed.

Written by the indexing model from the issue text.