refactor: 优化 URL 路径拼接，修复 /v1 重复问题

## 主要变更

**核心修改**：
- 路由定义：/:protocol/v1/*path → /:protocol/*path
- proxy_handler：nativePath 直接使用 path 参数，不添加 /v1 前缀
- OpenAI 适配器：DetectInterfaceType 和 BuildUrl 去掉 /v1 前缀
- Anthropic 适配器：保持 /v1 前缀（Claude Code 兼容）

**URL 格式变化**：
- OpenAI: /openai/v1/chat/completions → /openai/chat/completions
- Anthropic: /anthropic/v1/messages (保持不变)

**base_url 配置**：
- OpenAI: 配置到版本路径，如 https://api.openai.com/v1
- Anthropic: 不配置版本路径，如 https://api.anthropic.com

## 测试验证

- 所有单元测试通过
- 所有集成测试通过
- 真实 API 测试验证成功
- 跨协议转换正常工作

## 文档更新

- 更新 backend/README.md URL 格式说明
- 同步 OpenSpec 规范文件

backend/internal/conversion/openai/adapter_test.go

@@ -28,12 +28,12 @@ func TestAdapter_DetectInterfaceType(t *testing.T) {
 		path     string
 		expected conversion.InterfaceType
 	}{
-		{"聊天补全", "/v1/chat/completions", conversion.InterfaceTypeChat},
-		{"模型列表", "/v1/models", conversion.InterfaceTypeModels},
-		{"模型详情", "/v1/models/gpt-4", conversion.InterfaceTypeModelInfo},
-		{"嵌入接口", "/v1/embeddings", conversion.InterfaceTypeEmbeddings},
-		{"重排序接口", "/v1/rerank", conversion.InterfaceTypeRerank},
-		{"未知路径", "/v1/unknown", conversion.InterfaceTypePassthrough},
+		{"聊天补全", "/chat/completions", conversion.InterfaceTypeChat},
+		{"模型列表", "/models", conversion.InterfaceTypeModels},
+		{"模型详情", "/models/gpt-4", conversion.InterfaceTypeModelInfo},
+		{"嵌入接口", "/embeddings", conversion.InterfaceTypeEmbeddings},
+		{"重排序接口", "/rerank", conversion.InterfaceTypeRerank},
+		{"未知路径", "/unknown", conversion.InterfaceTypePassthrough},
 	}
 
 	for _, tt := range tests {
@@ -53,11 +53,11 @@ func TestAdapter_BuildUrl(t *testing.T) {
 		interfaceType conversion.InterfaceType
 		expected     string
 	}{
-		{"聊天", "/v1/chat/completions", conversion.InterfaceTypeChat, "/v1/chat/completions"},
-		{"模型", "/v1/models", conversion.InterfaceTypeModels, "/v1/models"},
-		{"嵌入", "/v1/embeddings", conversion.InterfaceTypeEmbeddings, "/v1/embeddings"},
-		{"重排序", "/v1/rerank", conversion.InterfaceTypeRerank, "/v1/rerank"},
-		{"默认透传", "/v1/other", conversion.InterfaceTypePassthrough, "/v1/other"},
+		{"聊天", "/chat/completions", conversion.InterfaceTypeChat, "/chat/completions"},
+		{"模型", "/models", conversion.InterfaceTypeModels, "/models"},
+		{"嵌入", "/embeddings", conversion.InterfaceTypeEmbeddings, "/embeddings"},
+		{"重排序", "/rerank", conversion.InterfaceTypeRerank, "/rerank"},
+		{"默认透传", "/other", conversion.InterfaceTypePassthrough, "/other"},
 	}
 
 	for _, tt := range tests {
@@ -118,13 +118,13 @@ func TestIsModelInfoPath(t *testing.T) {
 		path     string
 		expected bool
 	}{
-		{"model_info", "/v1/models/gpt-4", true},
-		{"model_info_with_dots", "/v1/models/gpt-4.1-preview", true},
-		{"models_list", "/v1/models", false},
-		{"nested_path", "/v1/models/gpt-4/versions", true},
-		{"empty_suffix", "/v1/models/", false},
-		{"unrelated", "/v1/chat/completions", false},
-		{"partial_prefix", "/v1/model", false},
+		{"model_info", "/models/gpt-4", true},
+		{"model_info_with_dots", "/models/gpt-4.1-preview", true},
+		{"models_list", "/models", false},
+		{"nested_path", "/models/gpt-4/versions", true},
+		{"empty_suffix", "/models/", false},
+		{"unrelated", "/chat/completions", false},
+		{"partial_prefix", "/model", false},
 	}
 
 	for _, tt := range tests {