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

py/services/preview_asset_service.py

@@ -9,7 +9,7 @@ from urllib.parse import urlparse
 
 from ..utils.constants import CARD_PREVIEW_WIDTH, PREVIEW_EXTENSIONS
 from ..utils.civitai_utils import rewrite_preview_url
-from ..utils.preview_selection import select_preview_media
+from ..utils.preview_selection import resolve_mature_threshold, select_preview_media
 from .settings_manager import get_settings_manager
 
 logger = logging.getLogger(__name__)
@@ -49,9 +49,13 @@ class PreviewAssetService:
         blur_mature_content = bool(
             settings_manager.get("blur_mature_content", True)
         )
+        mature_threshold = resolve_mature_threshold(
+            {"mature_blur_level": settings_manager.get("mature_blur_level", "R")}
+        )
         first_preview, nsfw_level = select_preview_media(
             images,
             blur_mature_content=blur_mature_content,
+            mature_threshold=mature_threshold,
         )
 
         if not first_preview:
@@ -216,4 +220,3 @@ class PreviewAssetService:
         if "webm" in content_type:
             return ".webm"
         return ".mp4"
-