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/PunktfunkKit/Video/StreamPump.swift

@@ -63,6 +63,14 @@ final class StreamPump {
             while alive, !token.isStopped {
                 alive = autoreleasepool { () -> Bool in
                 do {
+                    // Background keep-alive: drain one AU to keep QUIC flow control + host pacing
+                    // healthy, then discard it BEFORE any decode/enqueue — no VideoToolbox/Metal work
+                    // off-screen. Skips all recovery/gate bookkeeping too; exitBackground requests a
+                    // fresh IDR and the re-anchor gate re-arms on the resumed frame-index gap.
+                    if connection.isVideoDropped {
+                        _ = try connection.nextAU(timeoutMs: 100)
+                        return true
+                    }
                     // Loss recovery (the primary path). Under the host's infinite GOP the only
                     // recovery keyframe is one we request. The reassembler drops unrecoverable AUs
                     // (framesDropped); the decoder then *conceals* the reference-missing deltas — a