Closes#103 (HIGH). Three attack surfaces on the import endpoint —
body.Dir, workspace.Template, workspace.FilesDir — were concatenated
via filepath.Join without validation, letting an unauthenticated
caller probe arbitrary filesystem paths with "../../../etc".
Two layers of defense:
1. resolveInsideRoot() rejects absolute paths and any relative path
whose lexically cleaned join escapes the provided root (Abs +
HasPrefix + separator guard). 6 tests cover happy path, traversal
attempts, absolute path, empty input, prefix-sibling escape, and
deep subpath resolution.
2. Route now runs behind middleware.AdminAuth so an unauthenticated
attacker can't reach the handler at all once a token exists.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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