Successful output reports size_bytes: 0 despite retrievable content

## Bug description

A successful v2 job returns a retrievable output file, but the output metadata reports `size_bytes: 0`. The same incorrect size is returned by both the job response and the asset metadata endpoint.

`hash: null` was also returned, although the contract may allow hashes to be computed lazily; the definite defect is the zero byte count for an available file.

## Live reproduction

Observed against:

- `comfy-api-proxy` `2e52c6e01448242a3e23b452c81f9099c1e77d90` (`v0.1.4`)
- ComfyUI `0.31.0` on a remote `mando:8188`
- DarkBeast/Krea2 API-format text-to-image workflow
- Proxy started with `--state-dir`

Job: `826efbdb-18c6-466d-a220-9783c92d0079`

1. Submit the workflow through `POST /api/v2/jobs`.
2. Poll `GET /api/v2/jobs/{id}` until `succeeded`.
3. Fetch the returned output asset with `GET /api/v2/assets/{asset_id}`.
4. Download `GET /api/v2/assets/{asset_id}/content`.

## Actual behavior

Both metadata responses report:

```json
{"size_bytes": 0, "hash": null}
```

The content endpoint returns HTTP 200 with:

- `Content-Length: 1553743`
- valid 1024×1024 RGB PNG
- SHA-256: `51e84d82990b906124249d6127aa57bb81e0683009ea42e5079c72d03a743514`

## Expected behavior

For a retrievable output, `size_bytes` should equal the actual content length. If output hashing is intentionally lazy, `hash: null` is acceptable, but `size_bytes: 0` should not be used for a known available file.

The job output metadata and `GET /api/v2/assets/{id}` should agree.

## Suspected cause

`src/comfy_api_proxy/app.py`, `_outputs()`, currently constructs output records with:

```python
"size_bytes": 0,
"hash": None,
```

The content handler can already retrieve the upstream bytes.

## Suggested regression test

For a successful fake-upstream job:

- fetch the output metadata;
- fetch the content;
- assert `size_bytes == len(content)`;
- assert job and asset metadata agree;
- allow `hash` to remain null if lazy hashing is intentional.

Reproduction harness and raw response capture are available locally if useful.