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/fixtures/testing_fixtures.py

@@ -8,8 +8,9 @@ This module provides fixtures for:
 - Additional testing utilities
 """
 
-import pytest
 from unittest.mock import Mock
+
+import pytest
 from sqlalchemy.exc import SQLAlchemyError
 
 
@@ -26,7 +27,7 @@ def empty_db(db):
         "inventory",  # Fixed: singular not plural
         "products",  # Referenced by products
         "vendors",  # Has foreign key to users
-        "users"  # Base table
+        "users",  # Base table
     ]
 
     for table in tables_to_clear: