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>

tests/unit/api/test_deps.py

@@ -52,8 +52,8 @@ from app.modules.tenancy.exceptions import (
     UnauthorizedStoreAccessException,
 )
 from app.modules.tenancy.models import User
+from app.modules.tenancy.schemas.auth import UserContext
 from middleware.auth import AuthManager
-from models.schema.auth import UserContext
 
 # ============================================================================
 # Fixtures

tests/unit/models/schema/test_auth.py

@@ -4,7 +4,7 @@
 import pytest
 from pydantic import ValidationError
 
-from models.schema.auth import (
+from app.modules.tenancy.schemas.auth import (
     UserCreate,
     UserLogin,
     UserResponse,

tests/unit/services/test_auth_service.py

@@ -8,7 +8,7 @@ from app.modules.tenancy.exceptions import (
     InvalidCredentialsException,
     UserNotActiveException,
 )
-from models.schema.auth import UserLogin
+from app.modules.tenancy.schemas.auth import UserLogin
 
 
 @pytest.mark.unit