fix(video): 修正默认码率配置并优化软件编码器

- 将默认码率从 8 Mbps 降至 1 Mbps，更适合嵌入式设备
- 修复 WebRtcConfig 中 max_bitrate < target_bitrate 的逻辑错误
- 优化 libx264/libx265 软件编码器的低延迟配置
- 优化 libvpx (VP8/VP9) 的实时编码参数

src/video/shared_video_pipeline.rs

@@ -77,7 +77,7 @@ impl Default for SharedVideoPipelineConfig {
             resolution: Resolution::HD720,
             input_format: PixelFormat::Yuyv,
             output_codec: VideoEncoderType::H264,
-            bitrate_kbps: 8000,
+            bitrate_kbps: 1000,
             fps: 30,
             gop_size: 30,
             encoder_backend: None,
@@ -311,7 +311,7 @@ impl SharedVideoPipeline {
             config.input_format
         );
 
-        let (frame_tx, _) = broadcast::channel(16);
+        let (frame_tx, _) = broadcast::channel(64);  // Increased from 16 for software encoding
         let (running_tx, running_rx) = watch::channel(false);
         let nv12_size = (config.resolution.width * config.resolution.height * 3 / 2) as usize;
         let yuv420p_size = nv12_size; // Same size as NV12