feat: 实现统一模型 ID 机制

实现统一模型 ID 格式 (provider_id/model_name)，支持跨协议模型标识和 Smart Passthrough。

核心变更：
- 新增 pkg/modelid 包：解析、格式化、校验统一模型 ID
- 数据库迁移：models 表使用 UUID 主键 + UNIQUE(provider_id, model_name) 约束
- Repository 层：FindByProviderAndModelName、ListEnabled 方法
- Service 层：联合唯一校验、provider ID 字符集校验
- Conversion 层：ExtractModelName、RewriteRequestModelName/RewriteResponseModelName 方法
- Handler 层：统一模型 ID 路由、Smart Passthrough、Models API 本地聚合
- 新增 error-responses、unified-model-id 规范

测试覆盖：
- 单元测试：modelid、conversion、handler、service、repository
- 集成测试：统一模型 ID 路由、Smart Passthrough 保真性、跨协议转换
- 迁移测试：UUID 主键、UNIQUE 约束、级联删除

OpenSpec：
- 归档 unified-model-id 变更到 archive/2026-04-21-unified-model-id
- 同步 11 个 delta specs 到 main specs
- 新增 error-responses、unified-model-id 规范文件

backend/internal/conversion/engine_supplemental_test.go

@@ -113,7 +113,7 @@ func TestConvertHttpResponse_CrossProtocol(t *testing.T) {
 
 	result, err := engine.ConvertHttpResponse(HTTPResponseSpec{
 		StatusCode: 200, Body: []byte(`{"id":"resp-1"}`),
-	}, "client", "provider", InterfaceTypeChat)
+	}, "client", "provider", InterfaceTypeChat, "")
 	require.NoError(t, err)
 	assert.Equal(t, 200, result.StatusCode)
 	assert.Contains(t, string(result.Body), "resp-1")
@@ -129,7 +129,7 @@ func TestConvertHttpResponse_DecodeError(t *testing.T) {
 	_ = engine.RegisterAdapter(providerAdapter)
 	_ = engine.RegisterAdapter(newMockAdapter("client", false))
 
-	_, err := engine.ConvertHttpResponse(HTTPResponseSpec{Body: []byte(`{}`)}, "client", "provider", InterfaceTypeChat)
+	_, err := engine.ConvertHttpResponse(HTTPResponseSpec{Body: []byte(`{}`)}, "client", "provider", InterfaceTypeChat, "")
 	assert.Error(t, err)
 }
 
@@ -189,7 +189,7 @@ func TestConvertHttpResponse_EmbeddingInterface(t *testing.T) {
 
 	result, err := engine.ConvertHttpResponse(HTTPResponseSpec{
 		StatusCode: 200, Body: []byte(`{"object":"list","data":[],"model":"test"}`),
-	}, "client", "provider", InterfaceTypeEmbeddings)
+	}, "client", "provider", InterfaceTypeEmbeddings, "")
 	require.NoError(t, err)
 	assert.NotNil(t, result)
 }
@@ -207,7 +207,7 @@ func TestConvertHttpResponse_RerankInterface(t *testing.T) {
 
 	result, err := engine.ConvertHttpResponse(HTTPResponseSpec{
 		StatusCode: 200, Body: []byte(`{"results":[],"model":"test"}`),
-	}, "client", "provider", InterfaceTypeRerank)
+	}, "client", "provider", InterfaceTypeRerank, "")
 	require.NoError(t, err)
 	assert.NotNil(t, result)
 }
@@ -242,7 +242,7 @@ func TestConvertHttpResponse_ModelsInterface(t *testing.T) {
 
 	result, err := engine.ConvertHttpResponse(HTTPResponseSpec{
 		StatusCode: 200, Body: []byte(`{"object":"list","data":[]}`),
-	}, "client", "provider", InterfaceTypeModels)
+	}, "client", "provider", InterfaceTypeModels, "")
 	require.NoError(t, err)
 	assert.NotNil(t, result)
 }
@@ -259,7 +259,7 @@ func TestConvertHttpResponse_ModelInfoInterface(t *testing.T) {
 
 	result, err := engine.ConvertHttpResponse(HTTPResponseSpec{
 		StatusCode: 200, Body: []byte(`{"id":"gpt-4","object":"model"}`),
-	}, "client", "provider", InterfaceTypeModelInfo)
+	}, "client", "provider", InterfaceTypeModelInfo, "")
 	require.NoError(t, err)
 	assert.NotNil(t, result)
 }