refactor: move capacity_forecast_service from billing to monitoring

Resolves the billing (core) → monitoring (optional) architecture violation
by moving CapacityForecastService to the monitoring module where it belongs.

- Create BillingMetricsProvider to expose subscription counts via stats_aggregator
- Move CapacitySnapshot model from billing to monitoring
- Replace direct MerchantSubscription queries with stats_aggregator calls
- Fix middleware test mocks to cover StoreDomain/MerchantDomain fallback chains

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

tests/unit/middleware/test_platform_context.py

@@ -312,7 +312,10 @@ class TestPlatformContextManager:
     def test_get_platform_from_domain_not_found(self):
         """Test domain lookup when platform not found."""
         mock_db = Mock(spec=Session)
-        mock_db.query.return_value.filter.return_value.filter.return_value.first.return_value = None
+        # Ensure all query chain variants return None for .first()
+        query_mock = mock_db.query.return_value
+        query_mock.filter.return_value.first.return_value = None
+        query_mock.filter.return_value.filter.return_value.first.return_value = None
 
         context = {"detection_method": "domain", "domain": "unknown.lu"}
 
@@ -367,8 +370,11 @@ class TestPlatformContextManager:
     def test_get_platform_inactive_not_returned(self):
         """Test that inactive platforms are not returned."""
         mock_db = Mock(spec=Session)
-        # First call returns None (is_active filter excludes it)
-        mock_db.query.return_value.filter.return_value.filter.return_value.first.return_value = None
+        # Ensure all query chain variants return None for .first()
+        # (primary Platform lookup and StoreDomain/MerchantDomain fallbacks)
+        query_mock = mock_db.query.return_value
+        query_mock.filter.return_value.first.return_value = None
+        query_mock.filter.return_value.filter.return_value.first.return_value = None
 
         context = {"detection_method": "domain", "domain": "inactive.lu"}
 

tests/unit/middleware/test_store_context.py

@@ -245,7 +245,13 @@ class TestStoreContextManager:
     def test_get_store_from_custom_domain_not_found(self):
         """Test custom domain not found in database."""
         mock_db = Mock(spec=Session)
-        mock_db.query.return_value.filter.return_value.filter.return_value.filter.return_value.first.return_value = None
+        # Ensure all query chain variants return None for .first()
+        # (primary StoreDomain lookup and MerchantDomain fallback)
+        query_mock = mock_db.query.return_value
+        query_mock.filter.return_value.first.return_value = None
+        query_mock.filter.return_value.filter.return_value.first.return_value = None
+        query_mock.filter.return_value.filter.return_value.filter.return_value.first.return_value = None
+        query_mock.filter.return_value.order_by.return_value.first.return_value = None
 
         context = {"detection_method": "custom_domain", "domain": "nonexistent.com"}