feat: Implement runner execution pipeline with planner dispatch and execution services

- Introduced RunnerBackgroundService to handle execution of runner segments.
- Added RunnerExecutionService for processing segments and aggregating results.
- Implemented PlannerQueueDispatchService to manage dispatching of planner messages.
- Created PlannerQueueDispatcherBackgroundService for leasing and processing planner queue messages.
- Developed ScannerReportClient for interacting with the scanner service.
- Enhanced observability with SchedulerWorkerMetrics for tracking planner and runner performance.
- Added comprehensive documentation for the new runner execution pipeline and observability metrics.
- Implemented event emission for rescan activity and scanner report readiness.

docs/ops/scheduler-worker-prometheus-rules.yaml

@@ -0,0 +1,42 @@
+groups:
+  - name: scheduler-worker
+    interval: 30s
+    rules:
+      - alert: SchedulerPlannerFailuresHigh
+        expr: sum(rate(scheduler_planner_runs_total{status="failed"}[5m]))
+          /
+          sum(rate(scheduler_planner_runs_total[5m])) > 0.05
+        for: 10m
+        labels:
+          severity: critical
+          service: scheduler-worker
+        annotations:
+          summary: "Planner failure ratio above 5%"
+          description: "More than 5% of planning runs are failing. Inspect scheduler logs and ImpactIndex connectivity before queues back up."
+      - alert: SchedulerPlannerLatencyHigh
+        expr: histogram_quantile(0.95, sum by (le) (rate(scheduler_planner_latency_seconds_bucket[5m]))) > 45
+        for: 10m
+        labels:
+          severity: warning
+          service: scheduler-worker
+        annotations:
+          summary: "Planner latency p95 above 45s"
+          description: "Planning latency p95 stayed above 45 seconds for 10 minutes. Check ImpactIndex, Mongo, or external selectors to prevent missed SLAs."
+      - alert: SchedulerRunnerBacklogGrowing
+        expr: max_over_time(scheduler_runner_backlog[15m]) > 500
+        for: 15m
+        labels:
+          severity: warning
+          service: scheduler-worker
+        annotations:
+          summary: "Runner backlog above 500 images"
+          description: "Runner backlog exceeded 500 images over the last 15 minutes. Verify runner workers, scanner availability, and rate limits."
+      - alert: SchedulerRunStuck
+        expr: sum(scheduler_runs_active) > 0 and max_over_time(scheduler_runs_active[30m]) == min_over_time(scheduler_runs_active[30m])
+        for: 30m
+        labels:
+          severity: warning
+          service: scheduler-worker
+        annotations:
+          summary: "Scheduler runs stuck without progress"
+          description: "Active runs count has remained flat for 30 minutes. Investigate stuck segments or scanner timeouts."