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/billing/routes/api/store_checkout.py

@@ -34,7 +34,7 @@ from app.modules.billing.schemas.billing import (
 from app.modules.billing.services import billing_service
 from app.modules.billing.services.subscription_service import subscription_service
 from app.modules.enums import FrontendType
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 store_checkout_router = APIRouter(
     dependencies=[Depends(require_module_access("billing", FrontendType.STORE))],