Backfill split_part(email,'@',1) trgm expression index once users table grows

MARS-376 S3 fuzzy_email redesign (mig021) moved the fuzzy_email comparison to local-part-only (split_part(email,'@',1)) to fix domain-suffix false-positive inflation. The existing users_email_trgm GIN index is built on the full email column and does NOT cover this new expression, so the fuzzy_email WHERE clause runs seq-scan at current ~486-row scale (correct/cheap for now, per PM ruling on mig021). Once users grows enough that seq-scan cost becomes material, add a GIN trgm expression index on split_part(email,'@',1) so the % operator engages it — same pattern as users_email_trgm/users_displayName_trgm from mig018.