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/admin_service.py

@@ -786,6 +786,22 @@ class AdminService:
     # PRIVATE HELPER METHODS
     # ============================================================================
 
+    def get_user_by_id(self, db: Session, user_id: int) -> User | None:
+        """
+        Get user by ID.
+
+        Public method for cross-module consumers that need to look up a user.
+        Returns None if not found (does not raise).
+
+        Args:
+            db: Database session
+            user_id: User ID
+
+        Returns:
+            User object or None
+        """
+        return db.query(User).filter(User.id == user_id).first()
+
     def _get_user_by_id_or_raise(self, db: Session, user_id: int) -> User:
         """Get user by ID or raise UserNotFoundException."""
         user = db.query(User).filter(User.id == user_id).first()