perf: speed up test suite with parallel execution

- Enable pytest-xdist with -n auto for parallel test execution
- Consolidate migrated_db fixture in conftest.py (was duplicated in 4 files)
- Remove local fixture definitions and unused imports from test files

Test execution: ~24s -> ~5s (~5x speedup)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pyproject.toml

@@ -51,4 +51,4 @@ pythonpath = ["src"]
 python_files = "test_*.py"
 python_classes = "Test*"
 python_functions = "test_*"
-addopts = "--durations=20"
+addopts = "--durations=20 -n auto"