When send_storage_backend=s3 the role previously deployed the container
with whatever was in send_s3_* (often empty strings from the defaults).
The container would then start, accept uploads, and fail to persist
anything silently. Same pattern as the validate blocks in coturn,
talk, bookstack and opnform: fail fast at task time with a clear error
that points at the four missing variables.
Skipped entirely when send_storage_backend=local (the default).
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