(feature) Add host backup control actions

Turn the host detail page into a more useful operator surface for
starting greenfield backups from Django.

Add quick actions for dry-run and real backup runs, keep the advanced
manual options available, and show whether a host is ready, disabled, or
blocked by missing global config. Surface queued and running counts plus
a direct link to the active run.

Expose requested backup options on the run detail page and cover the new
control flow with view tests.

src/pobsync_backend/forms.py

@@ -118,16 +118,19 @@ class GlobalConfigForm(forms.ModelForm):
 
 class ManualBackupForm(forms.Form):
     dry_run = forms.BooleanField(
+        label="Dry run",
         required=False,
         initial=True,
         help_text="Queue rsync in dry-run mode without writing a snapshot.",
     )
     prune = forms.BooleanField(
+        label="Apply retention after success",
         required=False,
         help_text="Apply retention after a successful non-dry-run backup.",
     )
-    prune_max_delete = forms.IntegerField(min_value=0, initial=10)
+    prune_max_delete = forms.IntegerField(label="Retention max delete", min_value=0, initial=10)
     prune_protect_bases = forms.BooleanField(
+        label="Protect base snapshots",
         required=False,
         help_text="Keep snapshots that are used as bases by other snapshots.",
     )