(feature) Show next scheduled run and backup run type in the UI

Add a scheduler helper that calculates the next due time for a cron-style
schedule expression and surface that value on the dashboard and host detail
pages.

Show the latest run type in host summaries and backup trend tables so
manual and scheduled backups are distinguishable in the Django UI.

Keep the calculation derived from existing ScheduleConfig data without
adding a migration.

src/pobsync_backend/views.py

@@ -32,6 +32,7 @@ from .host_ops import collect_host_checks, ensure_host_directories
 from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord, SshCredential
 from .retention import run_sql_retention_apply, run_sql_retention_plan
 from .self_check import collect_self_checks, summarize_self_checks
+from .scheduler import next_due_after
 from .snapshot_discovery import discover_snapshots, inspect_snapshot_discovery
 from .ssh_keys import SshKeyError, delete_generated_key_files, generate_ssh_key, merge_known_hosts, scan_known_host
 from .stats_summary import collect_dashboard_stats, collect_host_stats
@@ -41,7 +42,8 @@ from .stats_summary import collect_dashboard_stats, collect_host_stats
 def dashboard(request):
     global_config = GlobalConfig.objects.filter(name="default").first()
     hosts = list(
-        HostConfig.objects.annotate(snapshot_count=Count("snapshots", distinct=True), run_count=Count("runs", distinct=True))
+        HostConfig.objects.select_related("schedule")
+        .annotate(snapshot_count=Count("snapshots", distinct=True), run_count=Count("runs", distinct=True))
         .order_by("host")
     )
     for host_config in hosts:
@@ -50,6 +52,7 @@ def dashboard(request):
             .order_by("-started_at", "-discovered_at", "-id")
             .first()
         )
+        host_config.next_run_at = _next_run_for_host(host_config)
     stats_summary = collect_dashboard_stats(hosts=hosts, global_config=global_config)
     context = {
         "hosts": hosts,
@@ -255,6 +258,7 @@ def create_host_config(request):
 @staff_member_required
 def host_detail(request, host: str):
     host_config = get_object_or_404(HostConfig, host=host)
+    schedule = _schedule_for_host(host_config)
     queued_runs = host_config.runs.filter(status=BackupRun.Status.QUEUED)
     running_runs = host_config.runs.filter(status=BackupRun.Status.RUNNING)
     active_run = host_config.runs.filter(
@@ -265,7 +269,8 @@ def host_detail(request, host: str):
     stats_summary = collect_host_stats(host=host_config, limit=10)
     context = {
         "host": host_config,
-        "schedule": _schedule_for_host(host_config),
+        "schedule": schedule,
+        "next_run_at": _next_run_for_schedule(schedule, host_config),
         "discovery": inspect_snapshot_discovery(host=host_config),
         "host_checks": host_checks,
         "host_check_summary": summarize_self_checks(host_checks),
@@ -548,6 +553,19 @@ def _schedule_for_host(host_config: HostConfig) -> ScheduleConfig | None:
         return None
 
 
+def _next_run_for_host(host_config: HostConfig):
+    return _next_run_for_schedule(_schedule_for_host(host_config), host_config)
+
+
+def _next_run_for_schedule(schedule: ScheduleConfig | None, host_config: HostConfig):
+    if schedule is None or not schedule.enabled or not host_config.enabled:
+        return None
+    try:
+        return next_due_after(schedule.cron_expr, timezone.localtime(timezone.now()))
+    except ValueError:
+        return None
+
+
 def _default_schedule_initial() -> dict[str, object]:
     return {
         "cron_expr": "15 2 * * *",