feat: 实现 OpenAI LLM 服务（SSE 流式解析 + 多模态消息构建）及 System Prompt 定义

backend/internal/ai/llm/prompt.go

@@ -0,0 +1,25 @@
+package llm
+
+import "strings"
+
+// BuildSystemPrompt 根据语言和细节级别构建系统提示词。
+func BuildSystemPrompt(language, detailLevel string) string {
+	isChinese := strings.HasPrefix(language, "zh")
+
+	var prompt strings.Builder
+	if isChinese {
+		prompt.WriteString("你是一个视觉助手。用户通过摄像头看到一个场景，并用语音向你提问。请用简洁自然的中文回答。如果涉及视觉描述，先说\"我看到……\"。回答控制在3-5句话以内，除非用户要求详细说明。")
+	} else {
+		prompt.WriteString("You are a visual assistant. The user sees a scene through their camera and asks questions by voice. Answer concisely and naturally. If describing visual content, start with 'I see...'. Keep answers to 3-5 sentences unless the user asks for detail.")
+	}
+
+	if detailLevel == "high" {
+		if isChinese {
+			prompt.WriteString("请提供更详细的视觉描述，包括颜色、位置、数量等细节。")
+		} else {
+			prompt.WriteString(" Provide detailed visual descriptions including colors, positions, quantities, and other details.")
+		}
+	}
+
+	return prompt.String()
+}