PUT /_api/files now runs raster uploads through mediapreview.dispatch
(temp file for format routing: pyvips, ffmpeg for HEIC/HEIF/AVIF),
storing the untouched original as <hash>.orig<ext> and serving the
AVIF derivative <hash>.avif in links. SVG/GIF and failed conversions
fall back to plain <hash><ext> storage. FileStore.delete removes the
whole hash pair. Adds mediapreview[standard] dependency.