webdav now owns the complete thumbnail pipeline: stat -> validate -> cache
check -> download -> preprocess -> generate -> cache -> respond. The
thumbnails service is used only as a stateless imagor-compatible image
resizer via its /unsafe push endpoint.
- New ThumbnailWorkflow drives the pipeline; space-scoped refs are anchored
at the space ResourceId and downloads authenticate with the user's
x-access-token.
- Preprocessing (audio/geogebra/text/gif) moves from the thumbnails service
into webdav (services/webdav/pkg/preprocessor).
- Thumbnail caching moves to webdav (memory/file/s3 backends, file cache off
by default); resolutions are snapped orientation-aware in webdav.
- Generator URL defaults to the local thumbnails service; new WEBDAV_* config
for generator, cache and resolutions.
- Acceptance tests updated for the new sizing/processor behavior.
