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/image.py

@@ -6,33 +6,6 @@ Pydantic schemas for image operations.
 from pydantic import BaseModel
 
 
-class ImageUrls(BaseModel):
-    """URLs for image variants."""
-
-    original: str
-    medium: str | None = None  # 800px variant
-    thumb: str | None = None  # 200px variant
-
-    # Allow arbitrary keys for flexibility
-    class Config:
-        extra = "allow"
-
-
-class ImageUploadResponse(BaseModel):
-    """Response from image upload."""
-
-    success: bool
-    image: dict | None = None
-    error: str | None = None
-
-
-class ImageDeleteResponse(BaseModel):
-    """Response from image deletion."""
-
-    success: bool
-    message: str
-
-
 class ImageStorageStats(BaseModel):
     """Image storage statistics."""