Add concurrency guards to the terraform and deploy-on-merge workflows so
two merges in quick succession can't run against the same state or the
same hosts at once (queue, never cancel an in-flight run).
Enable native S3 state locking (use_lockfile) on the Backblaze B2 backend,
which needs OpenTofu 1.10+, so bump the CI tofu version 1.9.0 -> 1.10.10
and the required_version constraint to >= 1.10.0.
