test: add tests for merchant dashboard metrics and fix invoice template location

Move invoice PDF template from app/templates/invoices/ to
app/modules/orders/templates/invoices/ where InvoicePDFService expects it.
Expand invoice PDF tests to validate template path and existence.

Add unit tests for get_merchant_metrics() in tenancy, billing, and
customer metrics providers. Add unit tests for StatsAggregatorService
merchant methods. Add integration tests for the merchant dashboard
stats endpoint.

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

app/modules/core/tests/unit/test_stats_aggregator.py

@@ -0,0 +1,106 @@
+# app/modules/core/tests/unit/test_stats_aggregator.py
+"""Unit tests for StatsAggregatorService merchant methods."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from app.modules.contracts.metrics import MetricValue
+from app.modules.core.services.stats_aggregator import StatsAggregatorService
+
+
+def _make_metric(key: str, value: int, category: str = "test") -> MetricValue:
+    return MetricValue(key=key, value=value, label=key, category=category)
+
+
+@pytest.mark.unit
+@pytest.mark.core
+class TestStatsAggregatorMerchant:
+    """Tests for StatsAggregatorService merchant aggregation methods."""
+
+    def setup_method(self):
+        self.service = StatsAggregatorService()
+
+    def test_get_merchant_dashboard_stats_aggregates_providers(self, db):
+        """Aggregates metrics from providers that implement get_merchant_metrics."""
+        provider_a = MagicMock()
+        provider_a.metrics_category = "billing"
+        provider_a.get_merchant_metrics.return_value = [
+            _make_metric("billing.active_subscriptions", 2, "billing"),
+        ]
+
+        provider_b = MagicMock()
+        provider_b.metrics_category = "tenancy"
+        provider_b.get_merchant_metrics.return_value = [
+            _make_metric("tenancy.total_stores", 3, "tenancy"),
+        ]
+
+        module_a = MagicMock()
+        module_a.code = "billing"
+        module_b = MagicMock()
+        module_b.code = "tenancy"
+
+        with patch.object(
+            self.service, "_get_enabled_providers", return_value=[(module_a, provider_a), (module_b, provider_b)]
+        ):
+            result = self.service.get_merchant_dashboard_stats(db, merchant_id=1, platform_id=1)
+
+        assert "billing" in result
+        assert "tenancy" in result
+        assert result["billing"][0].value == 2
+        assert result["tenancy"][0].value == 3
+
+    def test_get_merchant_dashboard_stats_skips_providers_without_method(self, db):
+        """Skips providers that don't have get_merchant_metrics."""
+        provider = MagicMock(spec=[])  # No attributes at all
+        provider.metrics_category = "legacy"
+        module = MagicMock()
+        module.code = "legacy"
+
+        with patch.object(
+            self.service, "_get_enabled_providers", return_value=[(module, provider)]
+        ):
+            result = self.service.get_merchant_dashboard_stats(db, merchant_id=1, platform_id=1)
+
+        assert result == {}
+
+    def test_get_merchant_dashboard_stats_handles_provider_error(self, db):
+        """Gracefully handles a provider raising an exception."""
+        provider = MagicMock()
+        provider.metrics_category = "broken"
+        provider.get_merchant_metrics.side_effect = RuntimeError("DB error")
+        module = MagicMock()
+        module.code = "broken"
+
+        with patch.object(
+            self.service, "_get_enabled_providers", return_value=[(module, provider)]
+        ):
+            result = self.service.get_merchant_dashboard_stats(db, merchant_id=1, platform_id=1)
+
+        assert result == {}
+
+    def test_get_merchant_stats_flat(self, db):
+        """Flattens categorized metrics into a single dict."""
+        provider = MagicMock()
+        provider.metrics_category = "tenancy"
+        provider.get_merchant_metrics.return_value = [
+            _make_metric("tenancy.total_stores", 5, "tenancy"),
+            _make_metric("tenancy.team_members", 12, "tenancy"),
+        ]
+        module = MagicMock()
+        module.code = "tenancy"
+
+        with patch.object(
+            self.service, "_get_enabled_providers", return_value=[(module, provider)]
+        ):
+            flat = self.service.get_merchant_stats_flat(db, merchant_id=1, platform_id=1)
+
+        assert flat["tenancy.total_stores"] == 5
+        assert flat["tenancy.team_members"] == 12
+
+    def test_get_merchant_stats_flat_empty(self, db):
+        """Returns empty dict when no providers have merchant metrics."""
+        with patch.object(self.service, "_get_enabled_providers", return_value=[]):
+            flat = self.service.get_merchant_stats_flat(db, merchant_id=1, platform_id=1)
+
+        assert flat == {}