The audio API was internally codenamed "Douanier" (Le Douanier Rousseau — a
customs-officer pun on edge auth). Renamed to "Fourier": the FFT is literally
the transform behind /spectrum and most of the feature stack, and central auth
is the platform’s job now, not ours — so the name should point at the signal
work, not the gate.

Internal-codename-only: the public contract (/audio/v1, gateway headers) is
untouched. Mechanical case-aware sweep across armada/api + the completed-archive,
plus hand-rewritten prose (the Rousseau attribution → Joseph Fourier; dropped the
customs-gate / 🛂 metaphors). Renames: douanier.py → fourier.py,
deploy/douanier-worker.service → deploy/fourier-worker.service. Env prefix
DOUANIER_* → FOURIER_* (config contract; safe — nech.pl is pre-users), dev venv
~/.virtualenvs/douanier → fourier (shebangs fixed). 78 tests green after.