(feature) queue manual backups from the Django host page

Add a staff-only manual backup form to host detail pages with safe
dry-run defaults and optional retention settings.

Queue manual BackupRun records through the existing worker-backed runner
path instead of executing backups inside the web request.

Validate disabled hosts, missing global config, and invalid methods with
view tests covering the new UI flow.

src/pobsync_backend/forms.py

@@ -117,6 +117,23 @@ class GlobalConfigForm(forms.ModelForm):
         return instance
 
 
+class ManualBackupForm(forms.Form):
+    dry_run = forms.BooleanField(
+        required=False,
+        initial=True,
+        help_text="Queue rsync in dry-run mode without writing a snapshot.",
+    )
+    prune = forms.BooleanField(
+        required=False,
+        help_text="Apply retention after a successful non-dry-run backup.",
+    )
+    prune_max_delete = forms.IntegerField(min_value=0, initial=10)
+    prune_protect_bases = forms.BooleanField(
+        required=False,
+        help_text="Keep snapshots that are used as bases by other snapshots.",
+    )
+
+
 class ScheduleConfigForm(forms.ModelForm):
     cron_expr = forms.CharField(
         label="Cron expression",