Optimize previously uploaded media #79364

What problem does this address?

Client-side media currently optimizes media only at upload time. Sites that predate the feature (or media uploaded before it was enabled) get no benefit. There is no way to re-process previously uploaded media to a more efficient format/size using the in-browser pipeline.

Adding Media Library integration and bulk optimization lets users optimize their existing media (e.g. convert legacy JPEG/PNG to WebP/AVIF, regenerate sub-sizes, compress) using the same client-side pipeline, with graceful fallback.

This is one of the capabilities Anne called out for the 7.1 cycle ("optimization of previously uploaded media") and is currently only listed as a "7.1 or later" roadmap bullet on the tracking issue #76756 ("Media Library integration and bulk optimization") without a dedicated issue.

Prior art

Optimization of existing/previously uploaded media is prototyped in media-experiments (block media panel "optimize" controls + upload-queue actions that fetch the existing file, re-process, and sideload).

Acceptance criteria

• Trigger optimization for an already-uploaded attachment (fetch original, re-process client-side, sideload result)
• Entry point from the block media panel for the selected attachment
• Reuse the existing @wordpress/upload-media queue, worker offloading, and capability gating
• Graceful fallback / skip when the browser cannot process a given item
• Clear progress feedback and per-item error handling
• e2e coverage

Part of #76756.

Out of scope

• Bulk action / Media Library integration to optimize many items

This ticket will not cover bulk optimization, we can work on that in a follow up or as part of the media library rebuild.

Screenshots / Screencast