(ui) Add schedule overview for dashboard drill-down

Add a staff-only schedules page with filters for host, enabled state, and
prune state, including next run and last scheduler state.

Wire the dashboard Schedules metric to the new overview so all primary
dashboard count cards have useful destinations.

Refs #23

src/pobsync_backend/views.py

@@ -203,6 +203,44 @@ def snapshots_list(request):
     return render(request, "pobsync_backend/snapshots_list.html", context)
 
 
+@staff_member_required
+def schedules_list(request):
+    enabled = request.GET.get("enabled", "").strip()
+    prune = request.GET.get("prune", "").strip()
+    host = request.GET.get("host", "").strip()
+    schedules = ScheduleConfig.objects.select_related("host").order_by("host__host")
+    if enabled == "yes":
+        schedules = schedules.filter(enabled=True)
+    elif enabled == "no":
+        schedules = schedules.filter(enabled=False)
+    if prune == "yes":
+        schedules = schedules.filter(prune=True)
+    elif prune == "no":
+        schedules = schedules.filter(prune=False)
+    if host:
+        schedules = schedules.filter(host__host=host)
+
+    schedule_rows = []
+    for schedule in schedules[:200]:
+        schedule_rows.append(
+            {
+                "schedule": schedule,
+                "next_run_at": _next_run_for_schedule(schedule, schedule.host),
+            }
+        )
+
+    context = {
+        "schedule_rows": schedule_rows,
+        "total_count": schedules.count(),
+        "hosts": HostConfig.objects.order_by("host"),
+        "selected_enabled": enabled,
+        "selected_prune": prune,
+        "selected_host": host,
+        "scheduler_timezone": timezone.get_current_timezone_name(),
+    }
+    return render(request, "pobsync_backend/schedules_list.html", context)
+
+
 @staff_member_required
 def purged_snapshots(request):
     host = request.GET.get("host", "").strip()