bug: safely delete backup directories and show directory sizes

[Serph91P]

Problem

Deleting a local backup directory currently calls file deletion and fails with Errno 21: Is a directory. Backup directory sizes are not shown.

Reproduction

DELETE /api/v1/storage/local/files?path=actual-stack-backup returns HTTP 500 for /backups/actual-stack-backup.

Required behavior

• safely delete files, symlinks, and directories below the configured storage root
• reject traversal, storage-root deletion, and symlink escapes
• return itemized outcomes for bulk deletion
• load recursive directory sizes asynchronously without blocking listing
• do not follow symlinks during size traversal

Acceptance

See Phase 1 in docs/plans/2026-07-11-storage-recovery-komodo-implementation.md and the approved design in PR #140.