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/tenancy/services/admin_service.py

@@ -107,18 +107,31 @@ class AdminService:
         per_page: int = 10,
         search: str | None = None,
         role: str | None = None,
+        scope: str | None = None,
         is_active: bool | None = None,
     ) -> tuple[list[User], int, int]:
         """
         Get paginated list of users with filtering.
 
+        Args:
+            scope: Optional scope filter. 'merchant' returns users who are
+                   merchant owners or store team members.
+
         Returns:
             Tuple of (users, total_count, total_pages)
         """
         import math
 
+        from app.modules.tenancy.models import Merchant, StoreUser
+
         query = db.query(User)
 
+        # Apply scope filter
+        if scope == "merchant":
+            owner_ids = db.query(Merchant.owner_user_id).distinct()
+            team_ids = db.query(StoreUser.user_id).distinct()
+            query = query.filter(User.id.in_(owner_ids.union(team_ids)))
+
         # Apply filters
         if search:
             search_term = f"%{search.lower()}%"