fix(bugcollect): V4 迁移 SQL 修复 ADD INDEX IF NOT EXISTS 语法

MySQL 不支持 ALTER TABLE ADD INDEX IF NOT EXISTS，改用 CREATE INDEX。
已在生产数据库手动修复并标记 Flyway 迁移成功。

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

xuqm-bugcollect-service/src/main/resources/db/migration/V4__bugcollect_v12.sql

@@ -34,7 +34,6 @@ CREATE TABLE IF NOT EXISTS log_sessions (
     INDEX idx_ls_started_at (started_at)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
--- Ensure event_id has a uniqueness constraint (was added as NULL in V3)
--- Use an index rather than UNIQUE to tolerate legacy NULL rows
-ALTER TABLE log_issue_events
-    ADD INDEX IF NOT EXISTS idx_lie_event_id (event_id);
+-- Ensure event_id has an index (was added as NULL in V3)
+-- Note: MySQL does not support ALTER TABLE ADD INDEX IF NOT EXISTS
+CREATE INDEX idx_lie_event_id ON log_issue_events (event_id);