feat: link backup runs to snapshot records

Add a nullable SnapshotRecord foreign key to BackupRun and populate it
when run_pobsync_backup records a completed or failed snapshot. Keep the
existing snapshot_path for audit compatibility while making run-to-snapshot
navigation explicit in the database and admin.

src/pobsync_backend/admin.py

@@ -50,9 +50,10 @@ class HostConfigAdmin(admin.ModelAdmin):
 
 @admin.register(BackupRun)
 class BackupRunAdmin(admin.ModelAdmin):
-    list_display = ("host", "run_type", "status", "started_at", "ended_at", "snapshot_path")
+    list_display = ("host", "run_type", "status", "started_at", "ended_at", "snapshot")
     list_filter = ("run_type", "status", "started_at")
-    search_fields = ("host__host", "snapshot_path")
+    search_fields = ("host__host", "snapshot_path", "snapshot__dirname", "snapshot__path")
+    autocomplete_fields = ("snapshot",)
 
 
 @admin.register(SnapshotRecord)