feat: consolidate media service, add merchant users page, fix metrics overlap

- Merge ImageService into MediaService with WebP variant generation,
  DB-backed storage stats, and module-driven media usage discovery
  via new MediaUsageProviderProtocol
- Add merchant users admin page with scoped user listing, stats
  endpoint, template, JS, and i18n strings (de/en/fr/lb)
- Fix merchant user metrics so Owners and Team Members are mutually
  exclusive (filter team_members on user_type="member" and exclude
  owner IDs) ensuring stat cards add up correctly
- Update billing and monitoring services to use media_service
- Update subscription-billing and feature-gating docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/cms/schemas/__init__.py

@@ -43,20 +43,17 @@ from app.modules.cms.schemas.media import (
     MediaListResponse,
     MediaMetadataUpdate,
     MediaUploadResponse,
+    MediaUsageItem,
     MediaUsageResponse,
     MessageResponse,
     MultipleUploadResponse,
     OptimizationResultResponse,
-    ProductUsageInfo,
     UploadedFileInfo,
 )
 
 # Image schemas
 from app.modules.cms.schemas.image import (
-    ImageDeleteResponse,
     ImageStorageStats,
-    ImageUploadResponse,
-    ImageUrls,
 )
 
 # Theme schemas
@@ -109,13 +106,10 @@ __all__ = [
     "MessageResponse",
     "MultipleUploadResponse",
     "OptimizationResultResponse",
-    "ProductUsageInfo",
+    "MediaUsageItem",
     "UploadedFileInfo",
     # Image
-    "ImageDeleteResponse",
     "ImageStorageStats",
-    "ImageUploadResponse",
-    "ImageUrls",
     # Theme
     "ThemeDeleteResponse",
     "ThemePresetListResponse",