fix(tasks): make CreateNextIteration atomically self-guarding

Folds the "is this still the latest row in its series?" check directly
into the INSERT as a single atomic INSERT...SELECT...WHERE NOT EXISTS
statement, instead of a separate SELECT-then-INSERT sequence. Closes a
narrow race where the completion trigger and the periodic due-date
check could both pass their "still latest" check before either had
inserted, producing two duplicate successor rows for the same series.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VTUSAEKfsPc6WGDq45yPHD

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