fix: resolve remaining CI failures for DB backends

- PostgreSQL/MySQL: delete soft-deleted row before re-inserting same
  schema to avoid unique constraint violation on (subject, fingerprint)
- Cassandra: filter out soft-deleted referrers in GetReferencedBy to
  match memory store behavior

Author: millerjp

internal/storage/cassandra/store.go

@@ -1250,6 +1250,14 @@ func (s *Store) GetReferencedBy(ctx context.Context, subject string, version int
 	var refs []storage.SubjectVersion
 	var ref storage.SubjectVersion
 	for iter.Scan(&ref.Subject, &ref.Version) {
+		// Filter out soft-deleted referrers (consistent with memory store behavior)
+		var deleted bool
+		if err := s.readQuery(
+			fmt.Sprintf(`SELECT deleted FROM %s.subject_versions WHERE subject = ? AND version = ?`, qident(s.cfg.Keyspace)),
+			ref.Subject, ref.Version,
+		).WithContext(ctx).Scan(&deleted); err != nil || deleted {
+			continue
+		}
 		refs = append(refs, ref)
 	}
 	if err := iter.Close(); err != nil {

internal/storage/mysql/store.go

@@ -498,6 +498,15 @@ func (s *Store) createSchemaAttempt(ctx context.Context, record *storage.SchemaR
 		return fmt.Errorf("failed to marshal ruleset: %w", err)
 	}
 
+	// If a soft-deleted row with the same fingerprint exists, remove it first
+	// to avoid violating the unique constraint on (subject, fingerprint).
+	if existingDeleted {
+		_, _ = tx.ExecContext(ctx,
+			"DELETE FROM `schemas` WHERE subject = ? AND fingerprint = ?",
+			record.Subject, record.Fingerprint,
+		)
+	}
+
 	// Insert schema - unique constraint on (subject, version) prevents duplicates
 	_, err = tx.ExecContext(ctx,
 		"INSERT INTO `schemas` (subject, version, schema_type, schema_text, fingerprint, created_at, metadata, ruleset) VALUES (?, ?, ?, ?, ?, ?, ?, ?)",

internal/storage/postgres/store.go

@@ -533,6 +533,15 @@ func (s *Store) createSchemaAttempt(ctx context.Context, record *storage.SchemaR
 		return fmt.Errorf("failed to marshal ruleset: %w", err)
 	}
 
+	// If a soft-deleted row with the same fingerprint exists, remove it first
+	// to avoid violating the unique constraint on (subject, fingerprint).
+	if existingDeleted {
+		_, _ = tx.ExecContext(ctx,
+			`DELETE FROM schemas WHERE subject = $1 AND fingerprint = $2`,
+			record.Subject, record.Fingerprint,
+		)
+	}
+
 	// Insert schema - unique constraint on (subject, version) prevents duplicates
 	_, err = tx.ExecContext(ctx,
 		`INSERT INTO schemas (subject, version, schema_type, schema_text, fingerprint, created_at, metadata, ruleset)