Phase 4: Indexing - Metadata Index Write

parent issue #21 

## Objective

Add the indexing layer after O3 upload is successful


## Tasks

- [ ] After O3 upload succeeds:
  - Extract metadata: `project_id`, `service`, `ts_start`, `ts_end`, `levels`, `tags`
  - Insert into `log_batches` table with `o3_object_key`
- [ ] Ensure atomicity (upload + index write)