feat: add systemd user services and timer units

- Create systemd user service files for both Go and Rust implementations
- Add comprehensive timer configurations: 30-minute, hourly, and daily schedules
- Include security settings, resource limits, and proper service dependencies
- Add justfile recipes for service management (install, enable, disable, status)
- Remove deprecated Makefile in favor of just-based build system
- Fix Rust compilation warnings in imap.rs

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

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

systemd/mail2couch-rs.timer

@@ -0,0 +1,17 @@
+[Unit]
+Description=Run mail2couch Rust Implementation every 30 minutes
+Documentation=https://kode.naiv.no/olemd/mail2couch
+Requires=mail2couch-rs.service
+
+[Timer]
+# Run every 30 minutes
+OnCalendar=*:0/30
+# Run 2 minutes after boot if we missed a scheduled run
+OnBootSec=2min
+# If the system was off, run shortly after startup
+Persistent=true
+# Add some randomization to avoid all users hitting servers simultaneously
+RandomizedDelaySec=300
+
+[Install]
+WantedBy=timers.target