`processing` background mode is declared but never used

**This blocks 1.0.0 release.**

`Info.plist` declares `UIBackgroundModes: [audio, processing]` and `BGTaskSchedulerPermittedIdentifiers`, but there is **zero** `BGTaskScheduler` usage anywhere in the codebase.

App Review routinely rejects apps declaring background modes they don't exercise. Given this is heading for the App Store, it's worth removing `processing` (and the `BGTaskSchedulerPermittedIdentifiers` array) unless there's a plan to use it. `audio` is genuinely used and should stay.

Inspect Info.plist and search the codebase for BGTaskScheduler usage. Remove the unused processing background mode and BGTaskSchedulerPermittedIdentifiers array while retaining audio. Done means the app configuration no longer declares processing without removing the active audio mode.

Written by the indexing model from the issue text.