What changed and why

Adds standalone image upscaling to the server, exposed on both API surfaces:

• POST /sdapi/v1/extra-single-image — synchronous, A1111-compatible. Accepts a
  base64 image plus upscaler_1, upscaler_2, upscaling_resize / target dims and
  returns {"image": "<base64-png>", "html_info": ""}.
• POST /sdcpp/v1/upscale — async job variant, polled via the existing
  GET /sdcpp/v1/jobs/{id} endpoint.

The shared perform_upscale() function handles:

• Built-in resamplers: None (passthrough copy), Lanczos (CatmullRom filter),
  Nearest (Box filter) via a new resize_sd_image() helper in media_io.
• Model-backed ESRGAN upscalers resolved from hires_upscalers_dir.
• Optional two-upscaler pixel blending (upscaler_2_visibility), guarded so it only
  runs when both outputs share identical dimensions.
• Post-upscale resize to exact target size when a model's native scale differs.

A new AsyncJobKind::Upscale variant and UpscaleJobRequest struct extend the
existing async job system without touching unrelated code paths.

Verification

Build with server examples enabled:

cmake -B build -DSD_BUILD_EXAMPLES=ON
cmake --build build --target sd-server

Smoke test — synchronous Lanczos upscale (2×):

IMG=$(base64 -w0 input.png)
curl -s -X POST http://localhost:8080/sdapi/v1/extra-single-image \
  -H 'Content-Type: application/json' \
  -d "{\"image\":\"$IMG\",\"upscaler_1\":\"Lanczos\",\"upscaling_resize\":2}" \
  | jq -r .image | base64 -d > output.png

Async upscale:

curl -s -X POST http://localhost:8080/sdcpp/v1/upscale \
  -H 'Content-Type: application/json' \
  -d "{\"image\":\"$IMG\",\"upscaler\":\"Lanczos\",\"scale\":2}" \
  | jq .poll_url
# then poll: GET /sdcpp/v1/jobs/{id}

Checklist

• I have read and confirmed this PR follows the contribution guidelines.