Sync Flux API documentation with upstream Docs

[Copilot]

Updates the Flux package to match the latest upstream Docs OpenAPI contract and integration guidance.

• OpenAPI

  • Adds flux/openapi/flux.json
  • Documents POST /flux/images and POST /flux/tasks, including authentication, parameters, task schemas, and responses

• Integration guides

  • Updates API token acquisition and shared-balance guidance
  • Aligns image-generation model/size support and examples
  • Documents task timing fields: started_at, finished_at, and elapsed
  • Replaces unstable external example URLs with Ace Data Cloud CDN examples
  • Updates embedded image markup for MDX compatibility

{
  "started_at": 1736304669.839,
  "finished_at": 1736304679.439,
  "elapsed": 9.6
}

• Fixes sync: update from Docs (be603c4 docs: sync from PlatformBackend [automated]) #758