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/jobs/JobRunner.kt

@@ -1,6 +1,7 @@
 package com.gzzn.omms.msgexchange.jobs
 
 import com.gzzn.omms.msgexchange.config.OperationDayProps
+import com.gzzn.omms.msgexchange.infra.metrics.JobActivity
 import com.gzzn.omms.msgexchange.processing.BackfillService
 import jakarta.inject.Singleton
 import java.time.Clock
@@ -23,6 +24,7 @@ class JobRunner(
     private val backfill: BackfillService,
     private val historySweep: HistorySweepJob,
     private val clock: Clock,
+    private val activity: JobActivity,
     operationDayProps: OperationDayProps,
 ) {
     private val log = org.slf4j.LoggerFactory.getLogger(JobRunner::class.java)
@@ -39,28 +41,46 @@ class JobRunner(
     fun start() {
         if (thread != null) return
         running = true
+        activity.started()
         thread = Thread.ofPlatform().name("msgx-jobs").daemon(true).start { loop() }
-        log.info("job runner started (backfill 30s, history daily 03:30 {})", zone)
+        log.info("job runner started (backfill {}s, history daily 03:30 {})", TICK_PERIOD.seconds, zone)
     }
 
     fun stop() {
         running = false
         thread?.interrupt()
         thread = null
+        activity.stopped()
     }
 
     internal fun loop() {
         while (running) {
             try {
-                backfill.sweep()
-                maybeHistorySweep()
+                tickOnce()
             } catch (e: InterruptedException) {
                 Thread.currentThread().interrupt()
                 return
-            } catch (e: Exception) {
-                log.warn("job tick failed: {}", e.message ?: e.javaClass.simpleName)
             }
-            if (running) runCatching { Thread.sleep(Duration.ofSeconds(30).toMillis()) }
+            if (running) runCatching { Thread.sleep(TICK_PERIOD.toMillis()) }
+        }
+    }
+
+    /**
+     * 跑一轮作业并上报心跳（拉出来单独可测，避免测试里 sleep）。
+     * 回填扫描的选中条数即"扫描积压"的即时值；失败只累计计数，不刷新心跳时刻。
+     */
+    internal fun tickOnce() {
+        val startedAt = clock.instant()
+        val startedNanos = System.nanoTime()
+        try {
+            val selected = backfill.sweep(startedAt)
+            maybeHistorySweep()
+            activity.tickFinished(startedAt, (System.nanoTime() - startedNanos) / 1_000_000, selected)
+        } catch (e: InterruptedException) {
+            throw e
+        } catch (e: Exception) {
+            activity.tickFailed()
+            log.warn("job tick failed: {}", e.message ?: e.javaClass.simpleName)
         }
     }
 
@@ -78,4 +98,9 @@ class JobRunner(
             }
         }
     }
+
+    private companion object {
+        /** 作业循环周期：回填扫描与历史作业共用的代码常量（reference 记为 `backfill-scan-period`）。 */
+        val TICK_PERIOD: Duration = Duration.ofSeconds(30)
+    }
 }