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/system/conftest.py

@@ -1,5 +1,5 @@
 # tests/system/conftest.py
 """System test specific fixtures."""
-import pytest
+
 
 # Add any system-specific fixtures here if needed

tests/system/test_error_handling.py

@@ -5,7 +5,7 @@ System tests for error handling across the LetzVendor API.
 Tests the complete error handling flow from FastAPI through custom exception handlers
 to ensure proper HTTP status codes, error structures, and client-friendly responses.
 """
-import json
+
 
 import pytest
 
@@ -377,9 +377,9 @@ class TestErrorHandling:
             # All error responses should have these fields
             required_fields = ["error_code", "message", "status_code"]
             for field in required_fields:
-                assert (
-                    field in data
-                ), f"Missing {field} in error response for {endpoint}"
+                assert field in data, (
+                    f"Missing {field} in error response for {endpoint}"
+                )
 
             # Details field should be present (can be empty dict)
             assert "details" in data