feat: 新增语音转文字（ASR）功能

- 后端：新增 /api/audio/transcribe 接口，调用 Ollama whisper 进行语音识别
- 前端：新增 AudioTranscribePage.vue 页面，支持 MP3/WAV/M4A/OGG/FLAC 等格式
- 注册路由、工具卡片、智能助手欢迎语更新

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

eai_agentplatform/backend-go/internal/web/errors.go

@@ -0,0 +1,37 @@
+package web
+
+import "net/http"
+
+// AppError 业务错误（对齐 Python core/errors.py 的错误码与状态码）
+type AppError struct {
+	StatusCode int
+	Code       string
+	Message    string
+}
+
+func (e *AppError) Error() string { return e.Message }
+
+func NewBadRequest(msg string) *AppError {
+	return &AppError{http.StatusBadRequest, "bad_request", msg}
+}
+func NewAuthError(msg string) *AppError {
+	return &AppError{http.StatusUnauthorized, "unauthorized", msg}
+}
+func NewForbiddenError(msg string) *AppError {
+	return &AppError{http.StatusForbidden, "forbidden", msg}
+}
+func NewNotFoundError(msg string) *AppError {
+	return &AppError{http.StatusNotFound, "not_found", msg}
+}
+func NewConflictError(msg string) *AppError {
+	return &AppError{http.StatusConflict, "conflict", msg}
+}
+func NewLLMNotConfigured(msg string) *AppError {
+	return &AppError{http.StatusNotImplemented, "llm_not_configured", msg}
+}
+func NewLLMError(msg string) *AppError {
+	return &AppError{http.StatusBadGateway, "llm_request_failed", msg}
+}
+func NewPointsExhausted(msg string) *AppError {
+	return &AppError{http.StatusPaymentRequired, "ai_points_exhausted", msg}
+}

eai_agentplatform/backend-go/internal/web/response.go

@@ -0,0 +1,17 @@
+package web
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// OK 成功响应信封 {"data": ..., "error": null, "message": "success"}
+func OK(c *gin.Context, data any) {
+	c.JSON(http.StatusOK, gin.H{"data": data, "error": nil, "message": "success"})
+}
+
+// Fail 错误响应信封 {"data": null, "error": ..., "message": ...}
+func Fail(c *gin.Context, e *AppError) {
+	c.JSON(e.StatusCode, gin.H{"data": nil, "error": e.Code, "message": e.Message})
+}