Convert a value between units — length, mass, volume, temperature, area, speed, pressure, energy, power, digital storage, time, angle, frequency and more. Unknown units or a conversion between incompatible measures (e.g. kg → km) are rejected as a clean 400.

List the supported units and their metadata (abbreviation, measure, system, singular/plural names), optionally filtered to a single measure — a discovery endpoint for choosing valid `from`/`to` values.