Fix slow /downstreams endpoint for high-fanout nodes

[shangyian]

Summary

The /downstreams endpoint was slow when querying nodes with many direct children. This is because for high fanout nodes, we're switching from the recursive CTE approach to BFS. The BFS path was a lot slower than the recursive CTE because it performs N queries per level, whereas the recursive CTE does traversal + eager loading in a single database query.

This PR raises fanout_threshold from 50 to 10000 so that the recursive CTE is used for virtually all cases. The BFS fallback is now only triggered for extremely large graphs.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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