Implement email forwarder

Add core IMAP forwarding logic with time-based searches and Message-ID
deduplication to prevent duplicates in destination mailbox.

Changes:
- Implement SINCE-based email search using last_run.txt for incremental forwarding
- Add Message-ID extraction and IMAP checks for deduplication
- Configure systemd user services with drop-in overrides for portability
- Integrate Uptime Kuma pings for monitoring
- Set up virtual environment for dependency isolation
- Update documentation and configuration templates

email_forwarder.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=Email Forwarder
+
+[Service]
+Type=oneshot
+WorkingDirectory=${PROJECT_DIR}
+ExecStart=${PROJECT_DIR}/venv/bin/python main.py
+ExecStartPost=/usr/bin/curl -fsS --retry 3 ${UPTIME_SUCCESS_URL}
+OnFailure=email_forwarder-fail-notify.service
+
+[Install]
+WantedBy=default.target