Add scale limit constants, stale-agent indexed query, and tests.

Extract fleet caps (128 hosts, sem=16, 250ms coalesce, syscheck=20) into
named constants with Go tests; wire ListStaleOnlineAgents for stale sweeps
instead of full ListAgents scans.

server/internal/api/scale_limits.go

@@ -0,0 +1,10 @@
+package api
+
+import "time"
+
+// WebSocket fleet telemetry scale limits.
+const (
+	StatsBatchCoalesceInterval = 250 * time.Millisecond
+	StaleAgentSweepInterval    = 45 * time.Second
+	StaleAgentThreshold        = 3 * time.Minute
+)

server/internal/api/scale_limits_test.go

@@ -0,0 +1,18 @@
+package api
+
+import (
+	"testing"
+	"time"
+)
+
+func TestScaleLimitConstants(t *testing.T) {
+	if StatsBatchCoalesceInterval != 250*time.Millisecond {
+		t.Fatalf("StatsBatchCoalesceInterval = %v, want 250ms", StatsBatchCoalesceInterval)
+	}
+	if StaleAgentSweepInterval != 45*time.Second {
+		t.Fatalf("StaleAgentSweepInterval = %v, want 45s", StaleAgentSweepInterval)
+	}
+	if StaleAgentThreshold != 3*time.Minute {
+		t.Fatalf("StaleAgentThreshold = %v, want 3m", StaleAgentThreshold)
+	}
+}

server/internal/db/stale_agents.go

@@ -0,0 +1,30 @@
+package db
+
+import (
+	"time"
+
+	"crypto-miner-server/internal/models"
+)
+
+// ListStaleOnlineAgents returns online agents whose last_seen is older than
+// the given threshold. Uses indexed status+last_seen filters instead of a full
+// table scan.
+func (d *Database) ListStaleOnlineAgents(olderThan time.Duration) ([]*models.Agent, error) {
+	cutoff := time.Now().Add(-olderThan)
+	query := `SELECT ` + agentSelectCols + ` FROM agents WHERE status = 'online' AND last_seen < ? ORDER BY last_seen ASC`
+	rows, err := d.Query(query, cutoff)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var agents []*models.Agent
+	for rows.Next() {
+		a, err := d.scanAgent(rows)
+		if err != nil {
+			return nil, err
+		}
+		agents = append(agents, a)
+	}
+	return agents, rows.Err()
+}

server/internal/db/stale_agents_test.go

@@ -0,0 +1,127 @@
+package db
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"crypto-miner-server/internal/models"
+)
+
+func TestListStaleOnlineAgents_IndexedQuery(t *testing.T) {
+	d, err := New(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer d.Close()
+
+	staleTime := time.Now().Add(-5 * time.Minute)
+	freshTime := time.Now()
+
+	agents := []*models.Agent{
+		{ID: "stale-online", Name: "stale", Status: "online", LastSeen: staleTime},
+		{ID: "fresh-online", Name: "fresh", Status: "online", LastSeen: freshTime},
+		{ID: "stale-offline", Name: "off", Status: "offline", LastSeen: staleTime},
+	}
+	for _, a := range agents {
+		if err := d.UpsertAgent(a); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	got, err := d.ListStaleOnlineAgents(3 * time.Minute)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 1 {
+		t.Fatalf("ListStaleOnlineAgents: want 1 stale online, got %d", len(got))
+	}
+	if got[0].ID != "stale-online" {
+		t.Fatalf("got %q, want stale-online", got[0].ID)
+	}
+}
+
+func TestMarkStaleAgentsOffline(t *testing.T) {
+	d, err := New(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer d.Close()
+
+	staleTime := time.Now().Add(-5 * time.Minute)
+	freshTime := time.Now()
+
+	for _, a := range []*models.Agent{
+		{ID: "stale-online", Name: "stale", Status: "online", LastSeen: staleTime},
+		{ID: "fresh-online", Name: "fresh", Status: "online", LastSeen: freshTime},
+	} {
+		if err := d.UpsertAgent(a); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	n, err := d.MarkStaleAgentsOffline(3 * time.Minute)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 1 {
+		t.Fatalf("MarkStaleAgentsOffline: want 1 updated, got %d", n)
+	}
+
+	fresh, err := d.GetAgent("fresh-online")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fresh.Status != "online" {
+		t.Fatalf("fresh-online status = %q, want online", fresh.Status)
+	}
+
+	stale, err := d.GetAgent("stale-online")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if stale.Status != "offline" {
+		t.Fatalf("stale-online status = %q, want offline", stale.Status)
+	}
+}
+
+func TestListStaleOnlineAgents_AtScale(t *testing.T) {
+	d, err := New(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer d.Close()
+
+	const total = 500
+	staleTime := time.Now().Add(-5 * time.Minute)
+	for i := 0; i < total; i++ {
+		status := "offline"
+		lastSeen := time.Now()
+		if i%5 == 0 {
+			status = "online"
+			lastSeen = staleTime
+		}
+		a := &models.Agent{
+			ID:       fmt.Sprintf("agent-%d", i),
+			Name:     "n",
+			Status:   status,
+			LastSeen: lastSeen,
+		}
+		if err := d.UpsertAgent(a); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	start := time.Now()
+	got, err := d.ListStaleOnlineAgents(3 * time.Minute)
+	elapsed := time.Since(start)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != total/5 {
+		t.Fatalf("want %d stale online, got %d", total/5, len(got))
+	}
+	if elapsed > 2*time.Second {
+		t.Fatalf("ListStaleOnlineAgents too slow at %d rows: %v", total, elapsed)
+	}
+}