Fix FK violation when hard-deleting namespaces with backfills

[shangyian]

Summary

DELETE /namespaces/{ns}/hard?cascade=true raised a ForeignKeyViolation when any node in the namespace had materialization backfill records. The bulk SQL delete cascaded through node to noderevision to materialization in the database, but backfill.materialization_id lacked ON DELETE CASCADE, so Postgres blocked the materialization delete.

This PR adds cascade to that FK, along with a migration to apply it to the existing database.

Test Plan

Added a regression test that creates a node w/ materialization w/ backfill and verifies that the namespace hard-delete succeeds.

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

Deployment Plan

[netlify]

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