feat(plugins): serve OpenAI Images API through task plugins

Add the synchronous openai_image host protocol so a task plugin can claim
POST /v1/images/generations and /v1/images/edits; the host submits the
task, polls it inside the request and lets the plugin render the OpenAI
image response. Unclaimed models still fall back to the HTTP image relay.

- Migrate Ali (Bailian) image models to the alibaba plugin and remove the
  ali channel image/wan adaptors and the qwen model setting
- Add Doubao Seedream image generation to the doubao plugin
- Support retainResult on routes and indexed multipart file references
- Settle image counts from upstream usage or image payloads; document the
  rule in the billing conventions and plugin API docs
- Show billing source and task artifacts in usage logs; rename the
  DoubaoVideo channel display name to Doubao and refresh the favicon