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.service

@@ -0,0 +1,36 @@
+[Unit]
+Description=mail2couch Rust Implementation - Email backup to CouchDB
+Documentation=https://kode.naiv.no/olemd/mail2couch
+After=network.target
+Wants=network.target
+
+[Service]
+Type=oneshot
+User=%i
+ExecStart=%h/bin/mail2couch-rs
+WorkingDirectory=%h/.config/mail2couch
+
+# Environment
+Environment=PATH=%h/bin:/usr/local/bin:/usr/bin:/bin
+Environment=RUST_LOG=info
+
+# Logging
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=mail2couch-rs
+
+# Security settings
+NoNewPrivileges=yes
+PrivateTmp=yes
+ProtectSystem=strict
+ProtectHome=read-only
+ReadOnlyPaths=/
+ReadWritePaths=%h/.config/mail2couch %h/.local/share/mail2couch
+
+# Resource limits
+MemoryHigh=512M
+MemoryMax=1G
+CPUQuota=50%
+
+[Install]
+WantedBy=default.target