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/media.py

@@ -164,22 +164,21 @@ class MediaMetadataUpdate(BaseModel):
 # ============================================================================
 
 
-class ProductUsageInfo(BaseModel):
-    """Information about product using this media."""
+class MediaUsageItem(BaseModel):
+    """Information about an entity using this media."""
 
-    product_id: int
-    product_name: str
-    usage_type: str  # main_image, gallery, variant, etc.
+    entity_type: str  # Defined by provider (e.g. "product")
+    entity_id: int
+    entity_name: str
+    usage_type: str  # Defined by provider (e.g. "main_image", "gallery")
 
 
 class MediaUsageResponse(BaseModel):
     """Response showing where media is being used."""
 
     media_id: int | None = None
-    products: list[ProductUsageInfo] = []
-    other_usage: list[dict[str, Any]] = []
+    usage: list[MediaUsageItem] = []
     total_usage_count: int = 0
-    message: str | None = None
 
 
 # ============================================================================