feat(risk-control): record matched keyword in keyword-block logs

The risk control center's moderation log records had no field for the
keyword that triggered a keyword block, so the admin UI couldn't show
which keyword was hit (only the application slog logged it).

- migration 156: add matched_keyword column to content_moderation_logs
- ContentModerationLog gains MatchedKeyword; set it on keyword block
- repo CreateLog/ListLogs persist and read the column
- frontend: show "命中关键词" inline in the log table and detail modal
- i18n: add matchedKeyword (zh/en)

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

frontend/src/i18n/locales/en.ts

@@ -2648,6 +2648,7 @@ export default {
       unbanFailed: 'Failed to unban user',
       inputDetailTitle: 'Input Summary Detail',
       inputDetailContent: 'Full Content',
+      matchedKeyword: 'Matched Keyword',
       queueDelay: 'Queued {ms} ms',
       allGroups: 'All Groups',
       allGroupsHint: 'Auditing all groups',

frontend/src/i18n/locales/zh.ts

@@ -2725,6 +2725,7 @@ export default {
       unbanFailed: '解封用户失败',
       inputDetailTitle: '输入摘要详情',
       inputDetailContent: '完整内容',
+      matchedKeyword: '命中关键词',
       queueDelay: '排队 {ms} ms',
       allGroups: '全部分组',
       allGroupsHint: '当前审计全部分组',