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/routes/api/admin_auth.py

@@ -21,9 +21,7 @@ from app.modules.core.services.auth_service import auth_service
 from app.modules.tenancy.exceptions import (
     InvalidCredentialsException,
 )
-from app.modules.tenancy.services.admin_platform_service import admin_platform_service
-from middleware.auth import AuthManager
-from models.schema.auth import (
+from app.modules.tenancy.schemas.auth import (
     LoginResponse,
     LogoutResponse,
     PlatformSelectResponse,
@@ -31,6 +29,8 @@ from models.schema.auth import (
     UserLogin,
     UserResponse,
 )
+from app.modules.tenancy.services.admin_platform_service import admin_platform_service
+from middleware.auth import AuthManager
 
 admin_auth_router = APIRouter(prefix="/auth")
 logger = logging.getLogger(__name__)