refactor(arch): move auth schemas to tenancy module and add cross-module service methods

Move all auth schemas (UserContext, UserLogin, LoginResponse, etc.) from
legacy models/schema/auth.py to app/modules/tenancy/schemas/auth.py per
MOD-019. Update 84 import sites across 14 modules. Legacy file now
re-exports for backwards compatibility.

Add missing tenancy service methods for cross-module consumers:
- merchant_service.get_merchant_by_owner_id()
- merchant_service.get_merchant_count_for_owner()
- admin_service.get_user_by_id() (public, was private-only)
- platform_service.get_active_store_count()

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

app/modules/tenancy/services/platform_service.py

@@ -142,6 +142,31 @@ class PlatformService:
             or 0
         )
 
+    @staticmethod
+    def get_active_store_count(db: Session, platform_id: int) -> int:
+        """
+        Get count of active stores on a platform.
+
+        Args:
+            db: Database session
+            platform_id: Platform ID
+
+        Returns:
+            Active store count
+        """
+        from app.modules.tenancy.models import Store
+
+        return (
+            db.query(func.count(StorePlatform.store_id))
+            .join(Store, Store.id == StorePlatform.store_id)
+            .filter(
+                StorePlatform.platform_id == platform_id,
+                Store.is_active == True,  # noqa: E712
+            )
+            .scalar()
+            or 0
+        )
+
     @staticmethod
     def get_platform_pages_count(db: Session, platform_id: int) -> int:
         """