Added tests/integration/targets-matrix.sh, which runs the full backup/restore
round trip through files_external's AmazonS3 and SFTP backend classes (S3
via a disposable s3mock container, SFTP via atmoz/sftp), since the existing
matrix only ever exercised the local backend.
This caught a real bug: moving a snapshot to the trash read it from the
location and streamed it straight back into a write on the same connection,
which SFTP can't do reliably (phpseclib fails to interleave a read and a
write on one connection). Backends now expose a native move() and the
id-bound snapshot object is moved with that instead of a read+write copy;
the path-bound database dump still goes through decrypt/re-encrypt, since
its AEAD authentication is tied to its path.
