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/billing/services/usage_service.py

@@ -14,12 +14,10 @@ and feature_service for limit resolution.
 import logging
 from dataclasses import dataclass
 
-from sqlalchemy import func
 from sqlalchemy.orm import Session
 
 from app.modules.billing.models import MerchantSubscription, SubscriptionTier
 from app.modules.billing.services.feature_aggregator import feature_aggregator
-from app.modules.tenancy.models import StoreUser
 
 logger = logging.getLogger(__name__)
 
@@ -222,12 +220,9 @@ class UsageService:
 
     def _get_team_member_count(self, db: Session, store_id: int) -> int:
         """Get active team member count for store."""
-        return (
-            db.query(func.count(StoreUser.id))
-            .filter(StoreUser.store_id == store_id, StoreUser.is_active == True)  # noqa: E712
-            .scalar()
-            or 0
-        )
+        from app.modules.tenancy.services.team_service import team_service
+
+        return team_service.get_active_team_member_count(db, store_id)
 
     def _calculate_usage_metrics(
         self, db: Session, store_id: int, subscription: MerchantSubscription | None