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/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()
+}