A podcast artifact is WAV; the bundled ffmpeg for MP3 is not built #2001

The gap

docs/architecture/studio.md lists this under Known gaps:

A podcast is WAV. The design encodes MP3 with a bundled ffmpeg, which is not built.

Why it matters

A generated episode is a single WAV joined from audio.cpp's output for each turn, so a twenty-minute podcast is a few hundred megabytes sitting in the user's data directory. Everything downstream pays for that: the artifact file, the download button, and anything the user does with the file afterwards. MP3 at the same length is an order of magnitude smaller, and it is what someone expects when they download a podcast rather than an audio asset.

Nothing else in the app is blocked by this, which is why it has sat open — but it is the difference between a Studio podcast being shareable and being a file people re-encode by hand.

Where

• surfsense_local/backend/modules/llm/providers/audiocpp/speech.py — synthesize() ends at SynthesizedAudio(joined_wav(voiced), "audio/wav"). This is where the joined audio exists as bytes and where an encode would sit.
• surfsense_local/backend/worker/studio/media/audio/podcast/pipeline.py — _EXTENSIONS already maps audio/mpeg to mp3, so the pipeline needs no change if the provider returns that media type. Reuse that rather than branching on format.
• surfsense_local/backend/worker/studio/shared/persist.py — its MIME-to-extension map decides the stored filename.
• surfsense_local/electron/scripts/audiocpp/stage.mjs and scripts/sdcpp/stage.mjs — the pattern to copy for staging a native binary: a pinned version, a checked SHA-256, a smoke run from the staged folder before it is swapped into place, and the upstream licence text carried alongside.
• surfsense_local/electron/electron-builder.yml — the extraResources list the staged folder has to join, and electron/src/main/sidecars/ — how the main process tells the backend where a staged binary lives, through SidecarContext.

What done looks like

• A podcast artifact is audio/mpeg, stored as .mp3, and the player and download in the artifact panel still work.
• A development checkout with nothing staged still produces a podcast. The other native runtimes degrade rather than fail (stage.mjs writes an empty folder and says why, and the app runs without that feature); WAV is the honest fallback here, so decide deliberately what happens when ffmpeg is absent and say so in the PR.
• The docs/architecture/packaging.md extraResources table gains its row, and the Studio doc's line "There is no Electron printToPDF and no ffmpeg in surfsense_local" is corrected along with the Voicing a podcast section. Delete the Known gaps line in the same PR.

What not to change

Settle the binary with a maintainer before you write the staging script. ffmpeg's licence depends on how it was configured — an LGPL build and a GPL build are different obligations for a distributed desktop app, and it is tens of megabytes on every installer for all three platforms. Which build, which pin, and whether it ships at all is a maintainer's call, not a contributor's; the code is the easy half. Comment here with what you propose before starting.

Do not reach for ffmpeg on the user's PATH as a shortcut. The app is offline-first and self-contained by design, and a feature that works only on machines that happen to have ffmpeg installed is worse than one that does not exist.

How to verify

surfsense_local/backend/tests/unit/worker/test_studio_podcast.py drives the pipeline with a fake voice provider, and tests/unit/llm/providers/audiocpp/test_speech.py covers synthesize() and the WAV joining. tests/packaging/test_audiocpp_voicing.py is the pattern for a test that needs the staged binary. Write the failing test first: a synthesized episode whose media type is audio/mpeg.

cd surfsense_local/backend
uv run pytest -m unit
uv run pytest -m integration
uv run ruff check .
cd ../.. && python scripts/check_docs.py

Before you start

Read CONTRIBUTING.md, the Studio doc and the packaging doc, which describes how every other native runtime is staged and frozen. Open your PR against dev, not main. Comment here to claim this.