refactor: inject config sources into scheduled backups

Introduce a ConfigSource interface so scheduled backups no longer need
to load host configuration directly from runtime YAML. Add a Django-backed
config source for SQL-driven backup runs, keep file-based config as the
CLI default, and make scheduled prune execution actually apply retention
after successful runs.

src/pobsync_backend/admin.py

@@ -35,6 +35,6 @@ class SnapshotRecordAdmin(admin.ModelAdmin):
 
 @admin.register(ScheduleConfig)
 class ScheduleConfigAdmin(admin.ModelAdmin):
-    list_display = ("host", "cron_expr", "enabled", "prune", "updated_at")
-    list_filter = ("enabled", "prune")
+    list_display = ("host", "cron_expr", "enabled", "prune", "last_status", "last_started_at", "updated_at")
+    list_filter = ("enabled", "prune", "last_status")
     search_fields = ("host__host", "cron_expr")