refactor: modernize code quality tooling with Ruff

- Replace black, isort, and flake8 with Ruff (all-in-one linter and formatter)
- Add comprehensive pyproject.toml configuration
- Simplify Makefile code quality targets
- Configure exclusions for venv/.venv in pyproject.toml
- Auto-fix 1,359 linting issues across codebase

Benefits:
- Much faster builds (Ruff is written in Rust)
- Single tool replaces multiple tools
- More comprehensive rule set (UP, B, C4, SIM, PIE, RET, Q)
- All configuration centralized in pyproject.toml
- Better import sorting and formatting consistency

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

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

tests/conftest.py

@@ -7,13 +7,8 @@ from sqlalchemy.pool import StaticPool
 
 from app.core.database import Base, get_db
 from main import app
-from models.database.inventory import Inventory
+
 # Import all models to ensure they're registered with Base metadata
-from models.database.marketplace_import_job import MarketplaceImportJob
-from models.database.marketplace_product import MarketplaceProduct
-from models.database.product import Product
-from models.database.user import User
-from models.database.vendor import Vendor
 
 # Use in-memory SQLite database for tests
 SQLALCHEMY_TEST_DATABASE_URL = "sqlite:///:memory:"