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/PunktfunkShared/DefaultsKeys.swift

@@ -112,6 +112,16 @@ public enum DefaultsKey {
     /// routed/VPN host), so connects go straight through instead of waiting out the wake timeout.
     /// The explicit "Wake Host" action stays available regardless. Read by ContentView.startSession.
     public static let autoWake = "punktfunk.autoWake"
+    /// iOS/iPadOS: keep a streaming session ALIVE when the app is backgrounded (audio background
+    /// mode). Off by default (today's freeze-on-background is the default). When on, backgrounding a
+    /// live session keeps audio playing and the QUIC/pump live while DROPPING video decode, and a
+    /// bounded timer (`backgroundTimeoutMinutes`) auto-disconnects if the user doesn't return. Read
+    /// by ContentView's scenePhase driver. Hidden on tvOS/macOS.
+    public static let backgroundKeepAlive = "punktfunk.backgroundKeepAlive"
+    /// iOS/iPadOS: minutes a backgrounded keep-alive session runs before auto-disconnecting (a
+    /// battery/thermal/bandwidth backstop). Default 10; the UI offers 1/5/10/30. The auto-disconnect
+    /// is non-deliberate (host linger kept), so a late return reconnects fast. Read on enterBackground.
+    public static let backgroundTimeoutMinutes = "punktfunk.backgroundTimeoutMinutes"
 }
 
 extension Notification.Name {