fix(server/reward): isDup 收窄到 unique/duplicate 防误判 FK/NOTNULL + 补 referrals 唯一路径测试

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/reward/store.go

@@ -152,5 +152,5 @@ func isDup(err error) bool {
 		return false
 	}
 	m := strings.ToLower(err.Error())
-	return strings.Contains(m, "unique") || strings.Contains(m, "duplicate") || strings.Contains(m, "constraint")
+	return strings.Contains(m, "unique") || strings.Contains(m, "duplicate")
 }