fix: renew tasks lock so long jobs are not stolen

[pplulee]

Summary

• The :tasks role is only renewed at the start of each run_tasks cycle. If a scheduled task (retention, CheckAllDNS, …) runs longer than 5 minutes, another worker steals the lock and runs the same work concurrently.
• Renew acquired_at every 30 seconds while tasks are running, using a stable locker name captured on the tasks thread.
• Treat Galera/MySQL StatementInvalid on lock updates as a failed acquire instead of crashing the tasks thread.

Addresses the lock-steal race described in #3595.

Test plan

• Run two workers and a long scheduled task (> 5 minutes).
• Confirm the second worker does not log acquired task role by stealing it from a lazy worker while the first is still running tasks.
• Confirm worker_roles.acquired_at keeps moving forward during the long task.
• Stop the holder; after ~5 minutes another worker can steal the role.

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