feat: add configurable mature blur threshold setting

Add new setting 'mature_blur_level' with options PG13/R/X/XXX to control
which NSFW rating level triggers blur filtering when NSFW blur is enabled.

- Backend: update preview selection logic to respect threshold
- Frontend: update UI components to use configurable threshold
- Settings: add validation and normalization for mature_blur_level
- Tests: add coverage for new threshold behavior
- Translations: add keys for all supported languages

Fixes #867

locales/ko.json

@@ -291,7 +291,15 @@
             "blurNsfwContent": "NSFW 콘텐츠 블러 처리",
             "blurNsfwContentHelp": "성인(NSFW) 콘텐츠 미리보기 이미지를 블러 처리합니다",
             "showOnlySfw": "SFW 결과만 표시",
-            "showOnlySfwHelp": "탐색 및 검색 시 모든 NSFW 콘텐츠를 필터링합니다"
+            "showOnlySfwHelp": "탐색 및 검색 시 모든 NSFW 콘텐츠를 필터링합니다",
+            "matureBlurThreshold": "[TODO: Translate] Mature Blur Threshold",
+            "matureBlurThresholdHelp": "[TODO: Translate] Set which rating level starts blur filtering when NSFW blur is enabled.",
+            "matureBlurThresholdOptions": {
+                "pg13": "[TODO: Translate] PG13 and above",
+                "r": "[TODO: Translate] R and above (default)",
+                "x": "[TODO: Translate] X and above",
+                "xxx": "[TODO: Translate] XXX only"
+            }
         },
         "videoSettings": {
             "autoplayOnHover": "호버 시 비디오 자동 재생",