Cache and prewarm Invidious proxy auto-detection

The proxy auto-detect path (when proxiesVideos is off) HEADed a
googlevideo URL with a 5 s timeout on every video. The verdict is a
property of the network, not the video, so the cost was paid for no
reason on videos 2..N. On a network where the CDN is blocked the full
5 s timeout was added to playback startup every single time.

Two changes:

1) ProxyDetectionCache (actor, per-instance, 10 min TTL). First miss
   pays the HEAD once and caches the verdict; subsequent videos hit
   the cache synchronously. Concurrent callers share one in-flight
   probe. The last-seen sample CDN URL is retained so future probes
   don't need a fresh URL from the current video.

2) PlayerService kicks off InvidiousAPI.prewarmProxyDetection() in
   parallel with the videoWith... API call. By the time streams come
   back, the verdict is usually already cached and proxyStreamsIfNeeded
   is a sync lookup. Cheap when there's nothing to prewarm.

Cache invalidation:
- on InstancesManager.update (URL change, proxy toggle flip)
- on InstancesManager.remove
- TTL covers the network-change case for now (no NWPathMonitor yet)

Yattee/Core/InstancesManager.swift

@@ -145,12 +145,17 @@ final class InstancesManager {
         }
 
         saveInstances()
+        Task { await ProxyDetectionCache.shared.invalidate(instance: instance) }
     }
 
     func update(_ instance: Instance) {
         if let index = instances.firstIndex(where: { $0.id == instance.id }) {
             instances[index] = instance
             saveInstances()
+            // Editing a source can change the proxy answer (URL change, toggle
+            // flip). Drop the cached auto-detect verdict so the next playback
+            // re-probes.
+            Task { await ProxyDetectionCache.shared.invalidate(instance: instance) }
         }
     }