fix: surface storage failures in /local/embed and /embed-upload

[nangelovv]

What

Make POST /local/embed and POST /embed-upload surface a storage failure (HTTP 500) instead of reporting success.

Why

Fixes #305. store_data_in_vector_db always returns a dict (with an "error" key on failure), but embed_local_file used if result: and embed_file_upload used if not result: — a failure dict is truthy / not-falsy, so both reported status:True / 200 while nothing was persisted.

Change

Both endpoints now treat not result or "error" in result as a failure and raise HTTP 500 with the error detail:

if not result or "error" in result:
    error_detail = (
        result["error"]
        if isinstance(result, dict) and isinstance(result.get("error"), str)
        else <endpoint default>
    )
    raise HTTPException(status_code=500, detail=error_detail)

POST /embed already performs this check and is unchanged. The success path is unchanged.

Testing

• New tests/test_main.py: test_embed_local_file_storage_error_returns_500 and test_embed_file_upload_storage_error_returns_500 (store returns an {"error": ...} dict → endpoint returns 500).
• Existing success-path embed tests still pass. tests/test_main.py: 12 passed.