Automatically optimize existing backgrounds

What's Changed

• Previously stored PNG and JPEG backgrounds are automatically converted to the 2048-pixel JPEG quality-85 policy when first requested, including backgrounds in the legacy storage location.
• Remember the normalized encoding in the stored filename to avoid repeated recompression, preserve visibility and transparency preferences, and bypass old browser image caches.
• Serialize migration with uploads and removal; retain unreadable originals without serving oversized or invalid images to browsers.
• Allow a bounded retry for transient SQLite file handles during integration-test cleanup; persistent cleanup failures still fail verification.

Release integrity

Native archives have matching platform-specific SPDX SBOM assets plus GitHub build-provenance and SBOM attestations. Installers and the built-in updater additionally verify a signed manifest and all packaged-file hashes.

Verification guide · Security policy