fix(billing): resolve 3 IMPORT-001 architecture violations in billing module

Replace direct imports from optional modules (catalog, orders, analytics)
with provider pattern calls (stats_aggregator, feature_aggregator) and
move usage_service from analytics to billing where it belongs.

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

app/modules/billing/services/__init__.py

@@ -41,6 +41,15 @@ from app.modules.billing.services.platform_pricing_service import (
     PlatformPricingService,
     platform_pricing_service,
 )
+from app.modules.billing.services.usage_service import (
+    UsageService,
+    usage_service,
+    UsageData,
+    UsageMetricData,
+    TierInfoData,
+    UpgradeTierData,
+    LimitCheckData,
+)
 
 __all__ = [
     "SubscriptionService",
@@ -63,4 +72,11 @@ __all__ = [
     "capacity_forecast_service",
     "PlatformPricingService",
     "platform_pricing_service",
+    "UsageService",
+    "usage_service",
+    "UsageData",
+    "UsageMetricData",
+    "TierInfoData",
+    "UpgradeTierData",
+    "LimitCheckData",
 ]