B4: probe task store, panel API, heartbeat ingestion, agent prober

- probe package: probe_ping_task/probe_ping_node tables; task CRUD
  with hub save semantics (id high-water, base-diff assignment,
  64-per-agent crowding refusal, auto_join cap); filing with
  assignment-gated insert; PingHistory weighted-median chart read
  over minute+hour tiers; auto-join on first Record (ensureRows).
- controller: /panel/api/agent/probe-tasks* management endpoints
  (node id space, hub validation chain verbatim); series=ping now
  serves real rows from probe.PingHistory; heartbeat ingests
  probeResults (missing latency skipped, batch capped); config
  response carries probe_tasks (always present, empty = clear).
- agent: per-task probe loops (immediate first probe, ticker,
  900ms resolve/dial budgets, <=3 addresses, resolve overrun = no
  sample), result buffer drained on heartbeat and requeued on
  failure; task set applied before the config-hash early return.

internal/web/service/probe/meta.go

@@ -143,6 +143,7 @@ func (s *Store) DeleteAgentData(agentID int64) error {
 	for _, table := range []string{
 		"probe_meta", "probe_traffic", "probe_metric",
 		"probe_metric_hour", "probe_ping_record", "probe_ping_hour",
+		"probe_ping_node",
 	} {
 		if _, err := tx.Exec(`DELETE FROM `+table+` WHERE agent_id = ?`, agentID); err != nil {
 			return err