The chunked video upload stored req.body.filename unmodified and later built
the merged path as path.join(tempDir, uploadMeta.filename). path.join does not
neutralise '../', so a filename like '../../uploads/logos/evil.svg' escaped the
temp dir on merge and overwrote arbitrary files. Requires admin with
photos.upload.
Fix: path.basename() the client filename in initializeUpload() and reject
names that collapse to nothing. Adds a regression test.
