Migrate format_presets.py to spec + code generation

[rtibbles]

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Overview

format_presets should be generated from a spec in the format #182 introduces. Studio, Kolibri and ricecooker read its field names and list order, so its public Python API should not change.

Complexity: Medium
Target branch: main

Context

• 32 presets. Constant names don't derive from ids (VIDEO_HIGH_RES = "high_res_video"), and each preset also has a <NAME>_READABLE constant used as its choices label.
• Six presets have a _READABLE constant that differs from their readable_name in PRESETLIST: video_dependency, audio_dependency, html5_dependency, h5p, qti, slideshow_image.
• VIDEO_VECTOR has constants but no record and no choices entry.
• ricecooker reads Preset.kind: https://github.com/learningequality/ricecooker/blob/d616dc6c3091acde9e1acab62f24fd1473d69712/ricecooker/utils/pipeline/extract_metadata.py#L24

The Change

• Where a preset's _READABLE constant and readable_name differ, the spec should hold both, so neither changes.
• js/FormatPresets.js should export the constants as its default export and the preset records as a named export.

Acceptance Criteria

• le_utils/constants/format_presets.py and js/FormatPresets.js are generated from spec/constants-format_presets.json, presetlookup.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 format_presets, including PRESETLIST order and RENDERABLE_PRESETS_ORDER.

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.