add migration to fix invalid SMTP settings caused by backfill edge case

migrations/20260729120000_[2.0.3]_fix_smtp_auth_backfill.down.sql

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+-- No-op: this migration repairs data and cannot be safely reversed. Rows that
+-- were originally unauthenticated are indistinguishable from repaired ones, so
+-- there is nothing to restore.

migrations/20260729120000_[2.0.3]_fix_smtp_auth_backfill.up.sql

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+-- Repair rows mis-classified by the [2.0.2] smtp_xoauth2 back-fill.
+-- That migration set smtp_authentication = 'login' whenever smtp_user and
+-- smtp_password were merely NOT NULL, but empty-string credentials (valid and
+-- common under 2.0.1, where SMTP authentication was optional) then fail the
+-- stricter SmtpSettings::is_configured() check. That in turn makes settings
+-- validation reject "gateway disconnect notifications" at startup and aborts
+-- the core process. Fall back to 'none' for those rows, restoring 2.0.1 behavior.
+UPDATE settings SET smtp_authentication = 'none'
+  WHERE smtp_authentication = 'login'
+    AND (smtp_user IS NULL OR smtp_user = ''
+      OR smtp_password IS NULL OR smtp_password = '');