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/orders/tests/unit/test_invoice_pdf_service.py

@@ -1,8 +1,13 @@
 """Unit tests for InvoicePDFService."""
 
+from pathlib import Path
+
 import pytest
 
-from app.modules.orders.services.invoice_pdf_service import InvoicePDFService
+from app.modules.orders.services.invoice_pdf_service import (
+    TEMPLATE_DIR,
+    InvoicePDFService,
+)
 
 
 @pytest.mark.unit
@@ -16,3 +21,25 @@ class TestInvoicePDFService:
     def test_service_instantiation(self):
         """Service can be instantiated."""
         assert self.service is not None
+
+    def test_template_directory_exists(self):
+        """Template directory must exist at the expected path."""
+        assert TEMPLATE_DIR.exists(), f"Template directory missing: {TEMPLATE_DIR}"
+        assert TEMPLATE_DIR.is_dir()
+
+    def test_invoice_template_exists(self):
+        """invoice.html template must exist in the template directory."""
+        template_path = TEMPLATE_DIR / "invoice.html"
+        assert template_path.exists(), f"Invoice template missing: {template_path}"
+
+    def test_template_can_be_loaded(self):
+        """Jinja2 environment can load the invoice template."""
+        template = self.service.env.get_template("invoice.html")
+        assert template is not None
+
+    def test_template_dir_is_inside_orders_module(self):
+        """Template directory should be inside the orders module, not the global templates."""
+        orders_module_dir = Path(__file__).parent.parent.parent
+        assert str(TEMPLATE_DIR).startswith(str(orders_module_dir)), (
+            f"TEMPLATE_DIR ({TEMPLATE_DIR}) should be inside orders module ({orders_module_dir})"
+        )