Fix phantom online agents: startup reset, init reconciliation, stale-sweep goroutine

server/internal/db/sqlite.go

@@ -169,6 +169,30 @@ func (d *Database) SetAgentOffline(id string) error {
 	return err
 }
 
+// MarkAllAgentsOffline resets every agent row to offline.  Called once at
+// server startup so rows left online by a previous crash are corrected
+// before any agent has had a chance to reconnect.
+func (d *Database) MarkAllAgentsOffline() error {
+	_, err := d.Exec("UPDATE agents SET status = 'offline' WHERE status = 'online'")
+	return err
+}
+
+// MarkStaleAgentsOffline marks agents offline when their last_seen timestamp
+// is older than the given staleness threshold.  Returns the number of rows
+// updated so the caller can emit a log line when something actually changed.
+func (d *Database) MarkStaleAgentsOffline(olderThan time.Duration) (int, error) {
+	cutoff := time.Now().Add(-olderThan)
+	res, err := d.Exec(
+		"UPDATE agents SET status = 'offline' WHERE status = 'online' AND last_seen < ?",
+		cutoff,
+	)
+	if err != nil {
+		return 0, err
+	}
+	n, _ := res.RowsAffected()
+	return int(n), nil
+}
+
 func (d *Database) DeleteAgent(id string) error {
 	_, err := d.Exec("DELETE FROM agents WHERE id = ?", id)
 	return err