refactor(arch): eliminate all cross-module model imports in service layer

Enforce MOD-025/MOD-026 rules: zero top-level cross-module model imports
remain in any service file. All 66 files migrated using deferred import
patterns (method-body, _get_model() helpers, instance-cached self._Model)
and new cross-module service methods in tenancy. Documentation updated
with Pattern 6 (deferred imports), migration plan marked complete, and
violations status reflects 84→0 service-layer violations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/orders/services/invoice_service.py

@@ -10,10 +10,12 @@ Handles:
 - PDF generation (via separate module)
 """
 
+from __future__ import annotations
+
 import logging
 from datetime import UTC, datetime
 from decimal import Decimal
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
 from sqlalchemy import and_, func
 from sqlalchemy.orm import Session
@@ -36,7 +38,9 @@ from app.modules.orders.schemas.invoice import (
     StoreInvoiceSettingsCreate,
     StoreInvoiceSettingsUpdate,
 )
-from app.modules.tenancy.models import Store
+
+if TYPE_CHECKING:
+    from app.modules.tenancy.models import Store
 
 logger = logging.getLogger(__name__)