feat(jobs): expose job heartbeat and scan backlog

新增 JobActivity 持有作业心跳与扫描积压，JobRunner 每轮 tick 上报（失败只计数、不刷新心跳），PipelineMetrics 注册 msgx.pipeline.job.heartbeat_age_seconds / ticks.total / failures.total / last_sweep_selected，新增 JobRunnerHealthIndicator（未启动 UP 并标 started=false；已启动但心跳缺失或超过 3×扫描周期 DOWN）。对应 reference 作业健康段与 [G-JOB-HEARTBEAT]。

验证：./gradlew test 145 tests / 0 fail / 0 skipped（新增 JobRunnerTest 2、JobRunnerHealthIndicatorTest 4、PipelineMetrics 指标注册 1）。

src/main/kotlin/com/gzzn/omms/msgexchange/infra/metrics/JobActivity.kt

@@ -0,0 +1,65 @@
+package com.gzzn.omms.msgexchange.infra.metrics
+
+import jakarta.inject.Singleton
+import java.time.Instant
+import java.util.concurrent.atomic.AtomicLong
+
+/**
+ * 维护作业的进程内活动事实（作业心跳、回填扫描积压），供 `/metrics` 与 `/health` 取用。
+ *
+ * 放在 `infra` 是为了让 `jobs`（写）与指标/健康（读）都零依赖：作业线程只更新这个持有者，
+ * 指标名与暴露方式由 [PipelineMetrics] 决定，存活判定由 `JobRunnerHealthIndicator` 决定。
+ * 计数在重启后归零；跨重启的累计值由指标后端聚合，这里不做持久化。
+ */
+@Singleton
+class JobActivity {
+    @Volatile
+    private var started = false
+
+    @Volatile
+    private var lastTickAt: Instant? = null
+
+    @Volatile
+    private var lastTickMillis = 0L
+
+    /** 上一轮回填扫描选中的待办条数（扫描积压的即时值）。 */
+    @Volatile
+    var lastSweepSelected = 0
+        private set
+
+    private val ticks = AtomicLong(0)
+    private val failures = AtomicLong(0)
+
+    fun started() {
+        started = true
+    }
+
+    fun stopped() {
+        started = false
+    }
+
+    /** 一次作业 tick 正常跑完：记录心跳时刻、耗时与本次扫描积压。 */
+    fun tickFinished(now: Instant, durationMillis: Long, sweepSelected: Int) {
+        lastTickAt = now
+        lastTickMillis = durationMillis
+        lastSweepSelected = sweepSelected
+        ticks.incrementAndGet()
+    }
+
+    /** 一次作业 tick 抛错：只累计失败，**不更新心跳时刻**——停摆由心跳年龄暴露。 */
+    fun tickFailed() {
+        failures.incrementAndGet()
+    }
+
+    fun snapshot(): Snapshot =
+        Snapshot(started, lastTickAt, lastTickMillis, lastSweepSelected, ticks.get(), failures.get())
+
+    data class Snapshot(
+        val started: Boolean,
+        val lastTickAt: Instant?,
+        val lastTickMillis: Long,
+        val lastSweepSelected: Int,
+        val ticks: Long,
+        val failures: Long,
+    )
+}

src/main/kotlin/com/gzzn/omms/msgexchange/infra/metrics/PipelineMetrics.kt

@@ -23,6 +23,9 @@ import java.time.Duration
  * - `msgx.pipeline.backfill.oldest_unmarked_seconds`：最老一条仍待自动回填的年龄
  * - `msgx.pipeline.watermark.lag`：水位落后信箱最新 ID 的距离
  * - `msgx.pipeline.hole.aged_out.total`：永久空洞放行次数
+ * - `msgx.pipeline.job.heartbeat_age_seconds`：距上一次作业 tick 完成的秒数（未跑过为 -1）
+ * - `msgx.pipeline.job.ticks.total` / `msgx.pipeline.job.failures.total`：作业 tick 完成/抛错次数
+ * - `msgx.pipeline.job.last_sweep_selected`：上一轮回填扫描选中的待办条数（扫描积压）
  *
  * 取数统一走 [BacklogSnapshotProvider]（30 秒 TTL），因此指标抓取不会打穿数据库。
  * 无法取数时以 `NaN` 上报（Micrometer 的惯例表示"本次无值"），而不是伪造 0。
@@ -41,6 +44,7 @@ class PipelineMetrics(
     private val cursor: BeanProvider<InboxCursorRepository>,
     private val mailbox: BeanProvider<CminmsgInboxRepository>,
     private val counters: PipelineCounters,
+    private val activity: JobActivity,
     private val clock: Clock,
 ) {
 
@@ -74,6 +78,23 @@ class PipelineMetrics(
         Gauge.builder("msgx.pipeline.late_arrival.detected.total", counters) { it.lateArrivalDetectedCount().toDouble() }
             .strongReference(true)
             .register(registry)
+
+        // 作业心跳与扫描积压：作业线程是回填的唯一驱动，停摆必须能被 /metrics 与 /health 看见。
+        Gauge.builder("msgx.pipeline.job.heartbeat_age_seconds", activity) { a ->
+            a.snapshot().lastTickAt?.let { Duration.between(it, clock.instant()).seconds.toDouble() } ?: -1.0
+        }.strongReference(true).register(registry)
+
+        Gauge.builder("msgx.pipeline.job.ticks.total", activity) { it.snapshot().ticks.toDouble() }
+            .strongReference(true)
+            .register(registry)
+
+        Gauge.builder("msgx.pipeline.job.failures.total", activity) { it.snapshot().failures.toDouble() }
+            .strongReference(true)
+            .register(registry)
+
+        Gauge.builder("msgx.pipeline.job.last_sweep_selected", activity) { it.snapshot().lastSweepSelected.toDouble() }
+            .strongReference(true)
+            .register(registry)
     }
 
     private fun backlogGauge(name: String, value: (com.gzzn.omms.msgexchange.infra.persistence.Backlog) -> Double) {