chore(db): add job_audit brin index

BRIN-индекс по created_at для retention-удалений.

Co-authored-by: Cursor <[email protected]>

migrations/postgres/000021_job_audit_brin.down.sql

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+DROP INDEX IF EXISTS idx_job_audit_created_brin;

migrations/postgres/000021_job_audit_brin.up.sql

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+CREATE INDEX IF NOT EXISTS idx_job_audit_created_brin
+    ON job_audit USING BRIN (created_at);

migrations/sqlite/000021_job_audit_brin.down.sql

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+DROP INDEX IF EXISTS idx_job_audit_created_brin;

migrations/sqlite/000021_job_audit_brin.up.sql

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+CREATE INDEX IF NOT EXISTS idx_job_audit_created_brin ON job_audit (created_at);