[BUG] DataPermissionServiceImpl.onSelectorCreate/onRuleCreated event listeners are not transactional and insert per-user in a loop

## Description
Both `@EventListener` methods, in the namespace-scoped branch, iterate `namespaceUserRelMapper.selectListByNamespaceId(namespaceId)` and call `dataPermissionMapper.insertSelective(...)` once **per user** with no `@Transactional`. A failure on the Nth insert leaves the first N-1 users granted and the rest not. No dedup against re-delivered events or concurrent creation (duplicates can be inserted).

## Location
```
shenyu-admin/.../service/impl/DataPermissionServiceImpl.java:267 and :305
```

## Impact
Inconsistent data permissions across users in the same namespace — some users see the new selector/rule, others do not, with no rollback.

## Suggested fix
Annotate both listeners `@Transactional(rollbackFor = Exception.class)` and switch to a single batch insert with an idempotency guard (`INSERT ... ON DUPLICATE KEY` or pre-check).

## Related existing issue(s)
None

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start in shenyu-admin/.../service/impl/DataPermissionServiceImpl.java at lines 267 and 305, and trace the namespace-scoped listener calls to namespaceUserRelMapper.selectListByNamespaceId and dataPermissionMapper.insertSelective. Verify that a failed insert rolls back all grants and that redelivered or concurrent events do not create duplicates; the work is done when permissions remain consistent for every namespace user.

Written by the indexing model from the issue text.