refactor: migrate to src-layout (PEP 517/518)

Move ses_monitor package into src/ directory for proper
Python packaging standards (src-layout).

- src/ses_monitor/ — package source
- pyproject.toml: [tool.setuptools.packages.find] where = ["src"]
- pyproject.toml: [tool.pytest.ini_options] pythonpath = ["src"]
- 59 tests pass unchanged

pyproject.toml

@@ -42,9 +42,11 @@ requires = ["setuptools>=61.0"]
 build-backend = "setuptools.build_meta"
 
 [tool.setuptools.packages.find]
+where = ["src"]
 include = ["ses_monitor*"]
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]
 python_files = ["test_*.py"]
+pythonpath = ["src"]