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/customer_fixtures.py

@@ -1,5 +1,6 @@
 # tests/fixtures/customer_fixtures.py
 import pytest
+
 from models.database.customer import Customer, CustomerAddress