feat: discover snapshots into Django records

Add a Django-native snapshot discovery service and management command
that scans backup directories, reads snapshot metadata, and idempotently
upserts SnapshotRecord rows. Expose it through the pobsync command
wrapper, update admin/docs, and cover discovery behavior with tests.

src/pobsync_backend/admin.py

@@ -57,9 +57,10 @@ class BackupRunAdmin(admin.ModelAdmin):
 
 @admin.register(SnapshotRecord)
 class SnapshotRecordAdmin(admin.ModelAdmin):
-    list_display = ("host", "kind", "dirname", "status", "started_at")
-    list_filter = ("kind", "status", "started_at")
+    list_display = ("host", "kind", "dirname", "status", "started_at", "discovered_at")
+    list_filter = ("kind", "status", "started_at", "discovered_at")
     search_fields = ("host__host", "dirname", "path")
+    readonly_fields = ("discovered_at",)
 
 
 @admin.register(ScheduleConfig)