style: apply black and isort formatting across entire codebase

- Standardize quote style (single to double quotes)
- Reorder and group imports alphabetically
- Fix line breaks and indentation for consistency
- Apply PEP 8 formatting standards

Also updated Makefile to exclude both venv and .venv from code quality checks.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/integration/api/v1/test_stats_endpoints.py

@@ -1,6 +1,7 @@
 # tests/integration/api/v1/test_stats_endpoints.py
 import pytest
 
+
 @pytest.mark.integration
 @pytest.mark.api
 @pytest.mark.stats
@@ -18,7 +19,9 @@ class TestStatsAPI:
         assert "unique_vendors" in data
         assert data["total_products"] >= 1
 
-    def test_get_marketplace_stats(self, client, auth_headers, test_marketplace_product):
+    def test_get_marketplace_stats(
+        self, client, auth_headers, test_marketplace_product
+    ):
         """Test getting marketplace statistics"""
         response = client.get("/api/v1/stats/marketplace", headers=auth_headers)