Adds full CRUD on mount contents, mirroring the P1 read pattern (handlers/
services classify; authorization stays in the service via the mount-root
folder grant; the provider does the I/O).
- mkdir / rename / delete / move-within branch inside FolderService and
FileManagementService (router injected into both)
- streaming upload via a new ExternalUploadService: the upload handler detects a
mount destination BEFORE the CAS ingest and streams the multipart body
straight to the provider (no BLAKE3/dedup). `write_stream` now takes a
lifetime-bound boxed stream so the borrowing multipart field can be passed
without buffering.
- deletes on mounts are permanent (no trash): the trash-first folder handler
routes `ext:` ids straight to the provider delete; file delete goes through the
branched delete_and_cleanup
- cross-backend move/copy (mount ↔ native, or between mounts) is forbidden
(UnsupportedOperation); the mount root itself cannot be renamed/moved/deleted
- every mutation emits a `target:"audit" event="external_mount.write"` line
- shared mount_dto builders synthesize FolderDto/FileDto from a provider MountStat
Tests: 529 unit + integration tests for mkdir/rename/delete, file rename/delete,
streaming upload, cross-boundary forbid, and stranger-denied — all against real
Postgres + a real provider (testcontainers).
Bradley Nelson