storage: fix DeleteTask atomicity and use recursive CTE

Replace BFS loop with a single recursive CTE to collect all descendant
task IDs in one query, and wrap all DELETE statements in a transaction
so a partial failure cannot leave orphaned executions.

Add TestDeleteTask_DeepSubtaskCascadeAtomic: creates a 3-level task
hierarchy with executions at each level, deletes the root, and verifies
all tasks and executions are removed with an explicit orphan-row check.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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