Migrate licenses.py to spec + code generation

[rtibbles]

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Overview

licenses should be generated from a spec in the format #182 introduces. It has no known wrong values, so its public Python API should not change.

Complexity: Low
Target branch: main

Context

• LICENSELIST ids are ints 1–9, while the constants hold license names (CC_BY = "CC BY"); licenselookup.json keys are the ids as strings.
• Every License.description is ""; licenselookup.json has no description field.

The Change

• js/Licenses.js should export the constants as its default export and the license records as a named export.

Acceptance Criteria

• le_utils/constants/licenses.py and js/Licenses.js are generated from spec/constants-licenses.json, licenselookup.json is deleted, and make build on a clean checkout leaves no diff.
• The pinned-API test from Add namedtuple spec support and migrate file_formats.py #182 passes unchanged for licenses.

AI usage

I made the decisions recorded on #181 and #182; Claude Code applied them to this module against the current code and drafted the issue.