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

email_forwarder/__init__.py

@@ -0,0 +1 @@
+# Email Forwarder Package