(feature) Add schedule editing view for hosts

Add a staff-only Django form for creating and updating host schedules using the
SQL-backed ScheduleConfig model. Link the form from host detail pages, validate
cron expressions with the existing scheduler parser, and preserve scheduler/CLI
behavior by writing to the same source of truth.

Cover default rendering, schedule creation, updates, and invalid cron handling
with view tests.

src/pobsync_backend/views.py

@@ -8,6 +8,7 @@ from django.views.decorators.http import require_POST
 
 from pobsync.errors import ConfigError
 
+from .forms import ScheduleConfigForm
 from .models import BackupRun, HostConfig, ScheduleConfig, SnapshotRecord
 from .retention import run_sql_retention_plan
 from .snapshot_discovery import discover_snapshots
@@ -95,8 +96,43 @@ def host_retention_plan(request, host: str):
     return render(request, "pobsync_backend/retention_plan.html", context)
 
 
+@staff_member_required
+def edit_host_schedule(request, host: str):
+    host_config = get_object_or_404(HostConfig, host=host)
+    schedule = _schedule_for_host(host_config)
+    if request.method == "POST":
+        form = ScheduleConfigForm(request.POST, instance=schedule)
+        if form.is_valid():
+            saved_schedule = form.save(commit=False)
+            saved_schedule.host = host_config
+            saved_schedule.save()
+            messages.success(request, f"Schedule saved for {host_config.host}.")
+            return redirect("host_detail", host=host_config.host)
+    else:
+        form = ScheduleConfigForm(instance=schedule, initial=_default_schedule_initial())
+
+    return render(
+        request,
+        "pobsync_backend/schedule_form.html",
+        {
+            "host": host_config,
+            "schedule": schedule,
+            "form": form,
+        },
+    )
+
+
 def _schedule_for_host(host_config: HostConfig) -> ScheduleConfig | None:
     try:
         return host_config.schedule
     except ScheduleConfig.DoesNotExist:
         return None
+
+
+def _default_schedule_initial() -> dict[str, object]:
+    return {
+        "cron_expr": "15 2 * * *",
+        "user": "root",
+        "enabled": True,
+        "prune_max_delete": 10,
+    }