(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/forms.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from django import forms
+
+from .models import ScheduleConfig
+from .scheduler import parse_cron_expr
+
+
+class ScheduleConfigForm(forms.ModelForm):
+    cron_expr = forms.CharField(
+        label="Cron expression",
+        help_text='Five-field cron expression, for example "15 2 * * *".',
+    )
+    prune_max_delete = forms.IntegerField(min_value=0)
+
+    class Meta:
+        model = ScheduleConfig
+        fields = (
+            "cron_expr",
+            "user",
+            "enabled",
+            "prune",
+            "prune_max_delete",
+            "prune_protect_bases",
+        )
+
+    def clean_cron_expr(self) -> str:
+        cron_expr = self.cleaned_data["cron_expr"].strip()
+        try:
+            parse_cron_expr(cron_expr)
+        except ValueError as exc:
+            raise forms.ValidationError(str(exc)) from exc
+        return cron_expr