(feature) Add cancellable backup runs and clearer dry-run logs

Add a cancel action for queued and running backup runs. Queued runs are
cancelled immediately, while running runs are marked for cancellation and
the worker terminates the active rsync process group.

Make dry-run log paths run-specific and add a defensive default dry-run
timeout so stuck dry-runs do not remain running indefinitely.

Remove rsync exit codes from run overview tables while keeping detailed
diagnostics available on the run detail payload.

src/pobsync_backend/views.py

@@ -10,6 +10,7 @@ from django.contrib.admin.views.decorators import staff_member_required
 from django.conf import settings
 from django.db.models import Count
 from django.shortcuts import get_object_or_404, redirect, render
+from django.utils import timezone
 from django.views.decorators.http import require_POST
 
 from pobsync.errors import PobsyncError
@@ -348,12 +349,37 @@ def run_detail(request, run_id: int):
     run = get_object_or_404(BackupRun.objects.select_related("host", "snapshot"), id=run_id)
     context = {
         "run": run,
+        "can_cancel": run.status in {BackupRun.Status.QUEUED, BackupRun.Status.RUNNING},
         "requested": run.result.get("requested") if isinstance(run.result, dict) else {},
         "result_json": _pretty_json(run.result),
     }
     return render(request, "pobsync_backend/run_detail.html", context)
 
 
+@staff_member_required
+@require_POST
+def cancel_run(request, run_id: int):
+    run = get_object_or_404(BackupRun.objects.select_related("host"), id=run_id)
+    if run.status not in {BackupRun.Status.QUEUED, BackupRun.Status.RUNNING}:
+        messages.warning(request, f"Run {run.id} is already {run.status}.")
+        return redirect("run_detail", run_id=run.id)
+
+    result = dict(run.result) if isinstance(run.result, dict) else {}
+    result["cancellation"] = {
+        "requested_at": timezone.now().isoformat(),
+        "previous_status": run.status,
+    }
+    update_fields = ["status", "result"]
+    run.status = BackupRun.Status.CANCELLED
+    run.result = result
+    if result["cancellation"]["previous_status"] == BackupRun.Status.QUEUED:
+        run.ended_at = timezone.now()
+        update_fields.append("ended_at")
+    run.save(update_fields=update_fields)
+    messages.success(request, f"Cancellation requested for run {run.id}.")
+    return redirect("run_detail", run_id=run.id)
+
+
 @staff_member_required
 def snapshot_detail(request, snapshot_id: int):
     snapshot = get_object_or_404(