feat(backfill): implement backfill tool for generating missing embeddings

configs/config.example.yaml

@@ -79,3 +79,11 @@ logging:
 observability:
   metrics_enabled: true
   pprof_enabled: false
+
+backfill:
+  enabled: false
+  run_on_startup: false
+  interval: "15m"
+  batch_size: 20
+  max_per_run: 100
+  include_archived: false