Initial import: nasmon health monitor, split out of home_nas

Go service (nasmon/) plus nfpm packaging (packaging/), carried over from
the home_nas monorepo. Adds rpm packaging alongside the existing deb
build (Fedora/RHEL/Rocky dependency names via nfpm overrides, per-format
systemd unit path, dpkg+rpm-compatible maintainer scripts) and a Gitea
Actions workflow that builds both formats on every push.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packaging/README.md

@@ -0,0 +1,41 @@
+# nasmon
+
+Lightweight NAS health monitor: web dashboard on port 8090 with CPU, memory
+(incl. ZFS ARC), temperatures, physical disks with SMART, ZFS pools and datasets;
+periodic SMART/ZFS checks with e-mail alerts.
+
+* Web UI:      http://<host>:8090
+* Settings:    http://<host>:8090/settings  (also /etc/nasmon/config.json)
+* State:       /var/lib/nasmon/nasmon.db  (SMART history, alert log)
+* Service:     systemctl status nasmon ; journalctl -u nasmon -f
+* JSON API:    /api/status, /api/history?hours=24, /api/disks/<serial>/history?days=30
+
+## ZFS datasets from the UI
+
+The ZFS page (http://<host>:8090/zfs) can create datasets in a pool — name (nested
+names like media/video are fine), owner uid:gid for the mountpoint, compression,
+optional quota — and destroy datasets that are empty (no data, no children, no
+snapshots; the check is repeated against the live system before "zfs destroy").
+
+nasmon runs sandboxed (ProtectSystem=strict → own mount namespace), so these
+commands are executed on the host through transient units:
+`systemd-run --wait --pipe --collect zfs create …`. Every action is logged
+(journalctl -u nasmon).
+
+nasmon has no authentication: keep port 8090 on the LAN only.
+
+## SMB / NFS shares from the UI
+
+http://<host>:8090/shares manages Samba shares (guest or per-user, files always
+written as the data owner uid 1000), SMB users (smbpasswd; a login-less Unix
+account is created when needed) and NFS exports (host/IP/CIDR clients, rw/ro,
+squash). Definitions are stored in /etc/nasmon/shares.json; nasmon regenerates
+/etc/samba/smb.conf (whole file, header "Managed by nasmon") and
+/etc/exports.d/nasmon.exports, validates with testparm, then reloads smbd /
+exportfs -ra. A rejected config is rolled back to the .bak copy. Daemons are
+enabled while at least one share exists and disabled otherwise.
+
+Alerts: the × on an open alert acknowledges it (hidden and not mailed until the
+problem changes or clears); the × on a resolved alert removes it from the log.
+Resolved alerts older than 7 days are hidden by default, older than 90 days
+deleted.