fix(stream): fix black screen on remote/Tailscale streaming

Three root-cause fixes for VLC showing a black screen when opening a
stream from a different network or via Tailscale:

1. PrioritizeTail: when VLC opens an MKV/MP4 stream it immediately seeks
   to the end of the file to read the container index (seekhead/moov
   atom). For active torrents those end-pieces aren't downloaded yet, so
   the reader blocks indefinitely. PrioritizeTail() opens a background
   reader positioned at the last 5 MB, keeping those pieces at high
   priority until ctx is cancelled or they finish downloading.

2. /health endpoint: GET /health returns a lightweight JSON response
   {"status":"ok","streaming":bool,...} so connectivity can be tested
   with a simple curl from any device before involving VLC.

3. Per-request logging: every incoming /stream request now logs the
   client IP and Range header, making it trivial to confirm whether
   remote/Tailscale clients are reaching the server at all.

internal/cmd/stream_handler.go

@@ -148,6 +148,13 @@ func handleStreamTask(parentCtx context.Context, at agent.Task, reporter *engine
 	task.StreamURL = srv.URLsJSON()
 	log.Printf("[%s] stream ready: %s (url: %s)", at.ID[:8], eng.FileName(), srv.URL())
 
+	// Pre-descargar los últimos 5 MB del archivo para que el moov atom (MP4)
+	// o el seekhead (MKV) estén disponibles cuando VLC los pida al abrir el
+	// stream. Sin esto, VLC busca el final del archivo, el lector bloquea
+	// esperando piezas no descargadas, y el resultado es pantalla negra en
+	// redes remotas donde la latencia amplifica el efecto.
+	eng.PrioritizeTail(ctx, 5*1024*1024)
+
 	// 5. Start watch progress reporter
 	if agentClient != nil {
 		watchReporter := engine.NewWatchReporter(agentClient, srv, at.ID)