Preserve image transparency metadata in app-server items

[copyberry]

Preserve image transparency metadata in app-server items

What changed

• Add transparentBackground to image-generation items and legacy completion events.
• Map Images API backgrounds to true for transparent, false for opaque, and null for automatic or unavailable values.
• Preserve the metadata in live notifications and persisted or resumed thread history while continuing to deserialize older items that omit it.

Testing

• Cover transparent, opaque, and automatic backgrounds, including image edits and thread read/resume flows.
• Verify the stable JSON shape and optional TypeScript field.

[github-actions]

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