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/contracts/cms.py

@@ -122,6 +122,22 @@ class ContentServiceProtocol(Protocol):
         ...
 
 
+@runtime_checkable
+class MediaUsageProviderProtocol(Protocol):
+    """Protocol for modules that track media file usage."""
+
+    def get_media_usage(self, db: "Session", media_id: int) -> list[dict]:
+        """Return list of usage records.
+
+        Each dict should contain:
+            entity_type: str (e.g. "product")
+            entity_id: int
+            entity_name: str
+            usage_type: str (e.g. "main_image", "gallery")
+        """
+        ...
+
+
 @runtime_checkable
 class MediaServiceProtocol(Protocol):
     """