[Feature Request] Add audio channel downmix option for transcoding (5.1ch to stereo)

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request. • Yes The feature TL;DR Immich preserves multi-channel audio during transcoding. When the source contains AC-3 5.1ch audio (e.g. MTS/AVCHD), the output becomes AAC 5.1ch, which has no audio playback on iOS (Safari / WKWebView / Immich iOS app). This is not a bug but a missing configuration option. Adding an optional stereo downmix setting (FFmpeg -ac 2) would: • Fix silent playback on iOS devices • Improve mobile/web compatibility • Reduce file size • Keep current behavior as default Proposed setting: Administration → Settings → Video → Transcoding → Audio Channels • Auto (preserve original) • Stereo (2ch) • 5.1 (6ch) Evidence: • AAC 5.1ch → ❌ iOS Immich app / Safari / Chrome (WebKit) • AAC stereo (downmixed) → ✅ works everywhere In short: add an optional audio channel downmix (st…