feat: 语音流式输入管线 + VAD前端集成 + 插件-工具合并清理

- 前端: VAD语音检测(@ricky0123/vad-web) + useVoiceInput双模式(流式WS/REST)
- Gateway: VoiceStreamManager代理WS流式STT到voice-service
- Voice-service: DashScope REST → Realtime WS → Whisper三级引擎 + ffmpeg转码
- 共享模块: pkg/audio(音频转换) + pkg/dashscope(ASR REST客户端)
- 清理: 移除旧plugin-manager和pkg/plugins，完成插件→工具合并
- 文档: 完善gateway-api.md和voice-service.md语音API文档
- 工具: scripts/voice/ 语音转换脚本集

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/voice-service/Dockerfile

@@ -19,7 +19,7 @@ RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /voice-service ./cmd/m
 # ========== 运行阶段 ==========
 FROM alpine:3.21
 
-RUN apk add --no-cache ca-certificates tzdata && \
+RUN apk add --no-cache ca-certificates tzdata ffmpeg && \
     cp /usr/share/zoneinfo/Asia/Shanghai /etc/localtime && \
     echo "Asia/Shanghai" > /etc/timezone