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/store.go

@@ -150,6 +150,29 @@ CREATE TABLE IF NOT EXISTS probe_ping_hour (
   hi       INTEGER,
   PRIMARY KEY (agent_id, ts, task_id)
 ) WITHOUT ROWID;
+
+-- Probe tasks and their per-agent assignment (hub "ping_task" /
+-- "ping_node"). B4 adds both. A task is one operator-maintained
+-- target — there is no preset list; region is the operator's own
+-- grouping label (empty = ungrouped). Ids come from a high-water
+-- mark in probe_setting so a deleted task's id is never reissued
+-- (hub next_id discipline): in-flight results and an editor opened
+-- before the delete must not land on a different task.
+CREATE TABLE IF NOT EXISTS probe_ping_task (
+  id       INTEGER PRIMARY KEY,
+  name     TEXT    NOT NULL,
+  target   TEXT    NOT NULL,
+  interval INTEGER NOT NULL DEFAULT 60,
+  auto_join INTEGER NOT NULL DEFAULT 0,
+  sort     INTEGER NOT NULL DEFAULT 0,
+  region   TEXT    NOT NULL DEFAULT ''
+);
+
+CREATE TABLE IF NOT EXISTS probe_ping_node (
+  task_id  INTEGER NOT NULL,
+  agent_id INTEGER NOT NULL,
+  PRIMARY KEY (task_id, agent_id)
+);
 `
 
 // Store is one open probe.db. The zero value is not usable; call Open.