## 1. Database Layer

- [x] 1.1 Add `get_document_by_hash(conn, content_hash)` function to `engine/kb/database.py` that returns `(document_id, title)` or `None`

## 2. Upload Endpoint

- [x] 2.1 Update `submit_job()` in `engine/kb/routes/jobs.py` to compute SHA256 hash of uploaded file bytes before staging
- [x] 2.2 Add duplicate check: call `get_document_by_hash()` and return HTTP 409 with `{"error": "duplicate", "document_id": <id>, "title": "<title>"}` if match found
- [x] 2.3 Apply same hash check for note submissions (hash the UTF-8 encoded note text)

## 3. Go Client

- [x] 3.1 Update `uploadFile()` in `client/cmd/add.go` to handle HTTP 409 responses — parse the JSON body and print "Already imported: <title> (doc ID: <id>)"
- [x] 3.2 Update recursive directory upload to continue on 409, track duplicate count, and include in summary output
- [x] 3.3 Handle 409 in JSON output mode — pass through the raw engine response

## 4. Testing

- [x] 4.1 Test: upload a file, then upload the same file again — verify 409 with correct document_id and title
- [x] 4.2 Test: upload a note, then upload the same note text — verify 409
- [x] 4.3 Test: upload a file, then upload a different file — verify 202 as normal
- [x] 4.4 Test: verify the worker-side `hash_exists()` safety net still works (direct job insertion bypassing API)