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/engine/stream_test.go

@@ -380,3 +380,31 @@ func (r *responseRecorder) ReadFrom(src io.Reader) (int64, error) {
 	n, err := io.Copy(r.body, src)
 	return n, err
 }
+
+// TestPrioritizeTail_SmallFile verifica que PrioritizeTail no lanza goroutine
+// cuando el archivo es demasiado pequeño (≤ 2×tailBytes).
+func TestPrioritizeTail_SmallFile(t *testing.T) {
+	s := &StreamEngine{
+		totalBytes: 5 * 1024 * 1024, // 5 MB — menor que 2×5 MB
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// No debe entrar en pánico ni bloquear con file == nil
+	s.PrioritizeTail(ctx, 5*1024*1024)
+	// Si llega aquí sin pánico, el test pasa
+}
+
+// TestPrioritizeTail_NilFile verifica que PrioritizeTail es seguro cuando
+// file es nil (engine no inicializado).
+func TestPrioritizeTail_NilFile(t *testing.T) {
+	s := &StreamEngine{
+		totalBytes: 100 * 1024 * 1024,
+		file:       nil,
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	s.PrioritizeTail(ctx, 5*1024*1024)
+	// No debe entrar en pánico
+}