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/api/admin/riskControl.ts

@@ -182,6 +182,7 @@ export interface ContentModerationLog {
   flagged: boolean
   highest_category: string
   highest_score: number
+  matched_keyword: string
   category_scores: Record<string, number>
   threshold_snapshot: Record<string, number>
   input_excerpt: string