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/ja.json

@@ -291,7 +291,15 @@
             "blurNsfwContent": "NSFWコンテンツをぼかす",
             "blurNsfwContentHelp": "成人向け（NSFW）コンテンツのプレビュー画像をぼかします",
             "showOnlySfw": "SFWコンテンツのみ表示",
-            "showOnlySfwHelp": "閲覧と検索時にすべてのNSFWコンテンツを除外します"
+            "showOnlySfwHelp": "閲覧と検索時にすべてのNSFWコンテンツを除外します",
+            "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": "ホバー時に動画を自動再生",