First slice of the Stories+Audiobook convergence (spec:
docs/specs/2026-06-13-stories-audiobook-maturity.md). Both features will compile
to one server-side chapterized renderer; this lands the shared pure builders and
wires them behind Audiobook.
New `backend/services/longform_render.py` (all pure, unit-tested without
ffmpeg/torch):
- build_ffmetadata(chapters, global_meta) — FFMETADATA1 with an optional global
tag block (title/author→artist/narrator→composer/year→date/genre/description→
comment) + chapter table.
- build_loudnorm_filter(preset) — `-af loudnorm` for ACX (~-19 LUFS, -3 dBTP) or
podcast (-16 LUFS); off/unknown → None. Opt-in, so default behavior stays
platform-identical.
- validate_cover_image — jpg/png + 8 MB cap guard.
- build_render_cmd — generalizes the m4b mux: m4b|mp3, optional cover
(attached_pic) + loudness, bitrate validated.
- build_concat_list — moved here.
`services/audiobook.py`: build_chapter_ffmetadata / build_m4b_cmd / build_concat_list
are now backward-compatible wrappers over the core (existing imports + tests
unchanged).
`POST /audiobook`: now accepts optional `format` (m4b|mp3), `loudness`,
`cover_path`, and `metadata` and passes them through — backend-complete; the UI
for these lands in PR 2.
Tests: tests/test_longform_render.py (28) + existing test_audiobook.py (11) green.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
