homehub: rsync-based home-drive sync via Synology hub

DESIGN.md is the rationale; README.md the operations half.
Four data classes: files (mtime-arbitrated two-way), git repos (manifest +
clone, never byte-synced), the browser link bar (union merge), and everything
else denied by default.

systemd/homehub-fresh.service

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+[Unit]
+Description=homehub freshness check: alerts only when a live machine has not synced for 24 h
+
+[Service]
+Type=oneshot
+# HH_QUIET is deliberately NOT set: the ok/stale lines belong in the journal, and
+# alert_send() uses DISPLAY+notify-send to reach the desktop when there is one.
+ExecStart=%h/homehub/bin/homehub fresh
+TimeoutStartSec=5min
+
+[Install]
+WantedBy=default.target

systemd/homehub-fresh.timer

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+[Unit]
+Description=homehub freshness check (half-hourly; the only thing that may complain)
+
+[Timer]
+OnBootSec=2min
+OnUnitActiveSec=30min
+RandomizedDelaySec=2min
+AccuracySec=1min
+Persistent=true
+
+[Install]
+WantedBy=timers.target

systemd/homehub-sync.service

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+[Unit]
+Description=homehub home-drive sync (rsync to NAS hub, mtime-arbitrated)
+After=network-online.target
+Wants=network-online.target
+Conflicts=sleep.target
+
+[Service]
+Type=oneshot
+# A background home sync must never make the machine stutter.
+Nice=19
+IOSchedulingClass=idle
+# Silent on success is a requirement, not an oversight (DESIGN.md §10).
+Environment=HH_QUIET=1
+ExecStart=%h/homehub/bin/homehub sync --exec
+TimeoutStartSec=3h
+# No retry storm: the next timer tick is the retry.
+
+[Install]
+WantedBy=default.target

systemd/homehub-sync.timer

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+[Unit]
+Description=homehub home-drive sync cycle (every 10 min, catches up after suspend)
+
+[Timer]
+OnBootSec=5min
+OnUnitActiveSec=10min
+RandomizedDelaySec=3min
+AccuracySec=1min
+Persistent=true
+
+[Install]
+WantedBy=timers.target