* ci: serialize infra runs and enable terraform state locking
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.
* ci: bump tofu to 1.10.10 in the validate workflow too
Missed this one in the last commit — the PR-time validate still pinned
1.9.0, which trips the new required_version >= 1.10.0 constraint.
* ci: drop use_lockfile — Backblaze B2 can't do native state locking
B2's S3 API returns 501 NotImplemented for the conditional PutObject that
use_lockfile relies on, so tofu plan/apply fails to acquire the lock.
Revert the lockfile and the 1.10 version bump it required; rely on the
concurrency guard to serialize applies instead. Left a note in the
backend block so this isn't re-attempted.
