(release) Track worker heartbeat for running jobs

Record worker pid, host, claim time, and heartbeat metadata on running
backup jobs so operators can see which worker owns a run.

Refresh the heartbeat while rsync is active and reconcile stale running
runs when the worker heartbeat stops. Add a worker option to tune or
disable stale-run reconciliation.

Refs #11

src/pobsync_backend/views.py

@@ -448,6 +448,7 @@ def run_detail(request, run_id: int):
     rsync_result = result.get("rsync") if isinstance(result.get("rsync"), dict) else {}
     failure = result.get("failure") if isinstance(result.get("failure"), dict) else {}
     prune_result = result.get("prune") if isinstance(result.get("prune"), dict) else {}
+    execution = result.get("execution") if isinstance(result.get("execution"), dict) else {}
     rsync_log_path = _run_rsync_log_path(run)
     rsync_log_tail = _run_rsync_log_tail(rsync_result, rsync_log_path)
     requested = result.get("requested") if isinstance(result.get("requested"), dict) else {}
@@ -455,6 +456,7 @@ def run_detail(request, run_id: int):
         "run": run,
         "can_cancel": run.status in {BackupRun.Status.QUEUED, BackupRun.Status.RUNNING},
         "requested": requested,
+        "execution": execution,
         "stats": run_stats if isinstance(run_stats, dict) else {},
         "rsync": rsync_result,
         "rsync_command": _run_rsync_command(rsync_result),