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/messaging/routes/api/admin_messages.py

@@ -47,7 +47,7 @@ from app.modules.messaging.services.message_attachment_service import (
     message_attachment_service,
 )
 from app.modules.messaging.services.messaging_service import messaging_service
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 admin_messages_router = APIRouter(prefix="/messages")
 logger = logging.getLogger(__name__)