feat(openai): 同步生图 API 支持并接入图片计费调度

- 同步 OpenAI 图片生成与编辑接口
- 接入图片请求解析、账号调度、转发与用量记录
- 接入图片计费与图片用量落库
- 限制 OAuth 生图仅支持无显式模型和尺寸的基础请求

backend/internal/handler/endpoint_test.go

@@ -25,12 +25,16 @@ func TestNormalizeInboundEndpoint(t *testing.T) {
 		{"/v1/messages", EndpointMessages},
 		{"/v1/chat/completions", EndpointChatCompletions},
 		{"/v1/responses", EndpointResponses},
+		{"/v1/images/generations", EndpointImagesGenerations},
+		{"/v1/images/edits", EndpointImagesEdits},
 		{"/v1beta/models", EndpointGeminiModels},
 
 		// Prefixed paths (antigravity, openai).
 		{"/antigravity/v1/messages", EndpointMessages},
 		{"/openai/v1/responses", EndpointResponses},
 		{"/openai/v1/responses/compact", EndpointResponses},
+		{"/openai/v1/images/generations", EndpointImagesGenerations},
+		{"/openai/v1/images/edits", EndpointImagesEdits},
 		{"/antigravity/v1beta/models/gemini:generateContent", EndpointGeminiModels},
 
 		// Gin route patterns with wildcards.
@@ -73,6 +77,8 @@ func TestDeriveUpstreamEndpoint(t *testing.T) {
 		{"openai responses nested", EndpointResponses, "/openai/v1/responses/compact/detail", service.PlatformOpenAI, "/v1/responses/compact/detail"},
 		{"openai from messages", EndpointMessages, "/v1/messages", service.PlatformOpenAI, EndpointResponses},
 		{"openai from completions", EndpointChatCompletions, "/v1/chat/completions", service.PlatformOpenAI, EndpointResponses},
+		{"openai image generations", EndpointImagesGenerations, "/v1/images/generations", service.PlatformOpenAI, EndpointImagesGenerations},
+		{"openai image edits", EndpointImagesEdits, "/openai/v1/images/edits", service.PlatformOpenAI, EndpointImagesEdits},
 
 		// Antigravity — uses inbound to pick Claude vs Gemini upstream.
 		{"antigravity claude", EndpointMessages, "/antigravity/v1/messages", service.PlatformAntigravity, EndpointMessages},