feat: expose IP-Adapter in server request schema and capabilities

[fszontagh]

Summary

The server could load an IP-Adapter model via --ip-adapter at startup, but the per-request JSON path never read the reference image or strength, so IP-Adapter was unusable from the server and the fields were absent from /sdcpp/v1/capabilities.

This wires the two per-request fields through the same path the existing control_image / control_strength fields use: ip_adapter_strength (number) and ip_adapter_image (3-channel image) are now parsed in from_json_str, advertised in the img_gen defaults and features, and documented in api.md. No new endpoints or parameters beyond mirroring the existing control-image pattern.

Related Issue / Discussion

Follow-up to #1803 (IP-Adapter support). Missing server support was reported in a comment on that PR.

Additional Information

Verified end-to-end against a running sd-server loaded with --ip-adapter ip-adapter_sd15.safetensors --clip_vision clip_vision_h.safetensors:

• GET /sdcpp/v1/capabilities now lists ip_adapter_image (features) and ip_adapter_strength (defaults).
• POST /sdcpp/v1/img_gen with a base64 ip_adapter_image completes with an image, and the server logs IP-Adapter: 4 image tokens, strength 1.00, confirming the request-supplied reference drives the adapter.

Checklist

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