feat(apple/M2): opt-in background keep-alive (audio + video-drop + timeout)

Backgrounding a live session no longer freezes it when the user opts in: audio
keeps playing (UIBackgroundModes audio), the QUIC connection + pump stay live,
video decode is DROPPED, and a bounded timer auto-disconnects. Off by default.

- PunktfunkConnection.setVideoDropped/isVideoDropped: a tiny lock-guarded flag
  both pumps read every iteration. StreamPump (stage-1), Stage2Pipeline (VT +
  PyroWave) drain nextAU() for flow control but DISCARD the AU before any
  VideoToolbox/Metal work — the crash/jetsam-safe seam (no GPU off-screen).
- SessionModel.enterBackground(timeoutMinutes:) / exitBackground(): set the drop
  flag, mute the mic (privacy — SessionAudio.setMicMuted pauses the capture
  engine), arm a DispatchSourceTimer that disconnect(deliberate:false)s on fire
  (keeps host linger → fast late reconnect). exitBackground clears the flag and
  requestKeyframe()s; the pump's freeze gate auto-arms on the resumed
  frame-index gap so concealed frames are withheld until the IDR re-anchors.
  disconnect() cancels the timer + clears isBackgrounded.
- ContentView scenePhase driver (iOS): .background+streaming+setting →
  enterBackground; .active → exitBackground. scenePhase (not willResignActive)
  so Control-Center/app-switcher peeks don't start the timer.
- Settings → General (iOS-only keepAliveSection): toggle + 1/5/10/30 timeout;
  new keys backgroundKeepAlive (def off) / backgroundTimeoutMinutes (def 10).
- Info.plist: UIBackgroundModes [audio] + NSSupportsLiveActivities (for M3).

macOS swift build + swift test green (142 tests). The iOS-gated scenePhase
handler + settings section are not exercised by the macOS CI target (known §9
gap) — need on-glass verification (audio never gaps, video re-anchors <1s LAN,
timeout ends the session, phone-call audio-steal degrades gracefully).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/ContentView.swift

@@ -96,6 +96,14 @@ struct ContentView: View {
     /// fires Wake-on-LAN up front and falls into the "Waking…" wait if the dial fails. Off: connects
     /// go straight through with no wake. The explicit "Wake Host" action is unaffected either way.
     @AppStorage(DefaultsKey.autoWake) private var autoWakeEnabled = true
+    /// Background keep-alive (Settings → General, iOS-only). Default OFF (today's freeze-on-background
+    /// is the default). When on, backgrounding a live session keeps audio + the connection alive and
+    /// drops video, auto-disconnecting after `backgroundTimeoutMinutes`.
+    @AppStorage(DefaultsKey.backgroundKeepAlive) private var backgroundKeepAlive = false
+    @AppStorage(DefaultsKey.backgroundTimeoutMinutes) private var backgroundTimeoutMinutes = 10
+    /// scenePhase drives the keep-alive: use THIS, not the willResignActive observers — resign-active
+    /// also fires for Control Center / app-switcher peeks, where the disconnect timer must not start.
+    @Environment(\.scenePhase) private var scenePhase
     private var gamepadUIActive: Bool {
         GamepadUIEnvironment.isActive(
             gamepadConnected: gamepadManager.active != nil, enabledSetting: gamepadUIEnabled)
@@ -122,6 +130,22 @@ struct ContentView: View {
         // tap uses, so trust policy / WoL / the approval sheet all come along. Never starts a
         // parallel session — this drives the one `model` ContentView owns.
         .onOpenURL { handleDeepLink($0) }
+        #if os(iOS)
+        // Background keep-alive driver (opt-in). Only .background/.active matter; .inactive (a
+        // transient peek) is ignored so the disconnect timer never starts for a Control-Center pull.
+        .onChange(of: scenePhase) { _, phase in
+            switch phase {
+            case .background:
+                if backgroundKeepAlive, model.phase == .streaming {
+                    model.enterBackground(timeoutMinutes: backgroundTimeoutMinutes)
+                }
+            case .active:
+                model.exitBackground()
+            default:
+                break
+            }
+        }
+        #endif
         .onChange(of: model.phase) { _, phase in
             switch phase {
             case .streaming: