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>

internal/history/ring.go

@@ -0,0 +1,59 @@
+// Package history keeps a fixed-size in-memory ring of metric samples.
+package history
+
+import (
+	"sync"
+	"time"
+)
+
+type Sample struct {
+	T       int64   `json:"t"` // unix seconds
+	CPU     float64 `json:"cpu"`
+	MemUsed uint64  `json:"mem_used"`
+	ARC     uint64  `json:"arc"`
+	CPUTemp float64 `json:"cpu_temp"`
+}
+
+type Ring struct {
+	mu   sync.RWMutex
+	buf  []Sample
+	head int
+	n    int
+}
+
+func New(size int) *Ring { return &Ring{buf: make([]Sample, size)} }
+
+func (r *Ring) Add(s Sample) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.buf[r.head] = s
+	r.head = (r.head + 1) % len(r.buf)
+	if r.n < len(r.buf) {
+		r.n++
+	}
+}
+
+// Since returns samples newer than t in chronological order, thinned to at
+// most maxPoints entries.
+func (r *Ring) Since(t time.Time, maxPoints int) []Sample {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	ts := t.Unix()
+	out := make([]Sample, 0, r.n)
+	start := (r.head - r.n + len(r.buf)) % len(r.buf)
+	for i := 0; i < r.n; i++ {
+		s := r.buf[(start+i)%len(r.buf)]
+		if s.T >= ts {
+			out = append(out, s)
+		}
+	}
+	if maxPoints > 0 && len(out) > maxPoints {
+		step := (len(out) + maxPoints - 1) / maxPoints
+		thin := out[:0:0]
+		for i := 0; i < len(out); i += step {
+			thin = append(thin, out[i])
+		}
+		out = thin
+	}
+	return out
+}