Fix v15 migration test to drop v19 table before replaying migrations

The v15 test pattern (delete migration records >= 15, drop schema, reopen to replay)
was failing because outreach_shared_number_overrides table from v19 migration was not
dropped before replaying. Add DROP TABLE IF EXISTS to the schema cleanup block to
match the same fix applied to v17 test.

This resolves the 'table already exists' error during v19 migration replay.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/db.test.mjs

@@ -150,6 +150,7 @@ test('v15 preserva linhas legadas ao migrar uma base populada em v14', () => {
       DROP TABLE outreach_campaign_queue;
       DROP TABLE outreach_campaign_variants;
       DROP TABLE outreach_campaigns;
+      DROP TABLE IF EXISTS outreach_shared_number_overrides;
       DELETE FROM schema_migrations WHERE version >= 15;
     `);
     db.close();