The backend no longer mentions MinIO anywhere: MinioClient -> GarageClient
(minio_client.py -> garage_client.py), MINIO_* env vars -> GARAGE_*, and
the `minio` Python package is replaced with `boto3` (AWS's generic S3
client) since Garage is the only storage backend this project targets
now. S3 API semantics are unchanged, so document_service.py's behavior
is identical — this is a naming and dependency swap, not a logic change.
Also updates test fixtures/names (FakeMinioClient -> FakeGarageClient,
FakeMinioSDK -> FakeS3SDK) and storage/README.md, which no longer frames
Garage as a MinIO replacement (that migration note has served its purpose
now that no MinIO reference remains in the codebase).
Backend: 54/54 tests passing. Frontend: 38/38 tests passing (unaffected,
verified for regressions since it talks to the backend only through the
unchanged REST API).
