Restructure as runnable Python module

Convert email_forwarder.py to a package with __main__.py for module execution.
Update systemd service to use -m email_forwarder.
Configure pyproject.toml to exclude state/ from packaging.

Changes:
- Create email_forwarder/ package with __init__.py and __main__.py
- Move main script to email_forwarder/email_forwarder.py
- Update ExecStart to python -m email_forwarder
- Add setuptools config to exclude state/ directory

pyproject.toml

@@ -7,4 +7,7 @@ version = "0.1.0"
 dependencies = [
     "imap-tools",
     "python-dotenv",
-]
+]
+
+[tool.setuptools.packages.find]
+exclude = ["state"]