* feat(destination): add WebDAV destination
Add a stdlib-only WebDAV destination mirroring the s3/azureblob flow:
bare-clone into a temp dir, optional zip, upload via HTTP PUT with Basic
auth, and prune stale remote files via PROPFIND/DELETE.
- types: WebDAVRepo struct, Destination.WebDAV field, Count()
- webdav: new package (UploadDirToWebDAV, DeleteObjectsNotInRepo)
- main: dispatch loop on par with azureblob
- conf.example.yml + gickup_spec.json: document the new destination
* feat: auto retry
Auto retry network errors and some situations.
Servers that disable Depth: infinity (Sabre/DAV, Nextcloud and others
answer 400/403/501) failed listing and could not prune stale files.
Try infinity once, fall back to recursive PROPFIND walk on rejection.
* refactor: use gowebdav client
Classify retry errors and make backoff interruptible.
