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.go

@@ -15,10 +15,12 @@ import (
 // ──────────────────────────────────────────────────────────
 
 const (
-	EndpointMessages        = "/v1/messages"
-	EndpointChatCompletions = "/v1/chat/completions"
-	EndpointResponses       = "/v1/responses"
-	EndpointGeminiModels    = "/v1beta/models"
+	EndpointMessages          = "/v1/messages"
+	EndpointChatCompletions   = "/v1/chat/completions"
+	EndpointResponses         = "/v1/responses"
+	EndpointImagesGenerations = "/v1/images/generations"
+	EndpointImagesEdits       = "/v1/images/edits"
+	EndpointGeminiModels      = "/v1beta/models"
 )
 
 // gin.Context keys used by the middleware and helpers below.
@@ -44,6 +46,10 @@ func NormalizeInboundEndpoint(path string) string {
 		return EndpointChatCompletions
 	case strings.Contains(path, EndpointMessages):
 		return EndpointMessages
+	case strings.Contains(path, EndpointImagesGenerations) || strings.Contains(path, "/images/generations"):
+		return EndpointImagesGenerations
+	case strings.Contains(path, EndpointImagesEdits) || strings.Contains(path, "/images/edits"):
+		return EndpointImagesEdits
 	case strings.Contains(path, EndpointResponses):
 		return EndpointResponses
 	case strings.Contains(path, EndpointGeminiModels):
@@ -69,6 +75,9 @@ func DeriveUpstreamEndpoint(inbound, rawRequestPath, platform string) string {
 
 	switch platform {
 	case service.PlatformOpenAI:
+		if inbound == EndpointImagesGenerations || inbound == EndpointImagesEdits {
+			return inbound
+		}
 		// OpenAI forwards everything to the Responses API.
 		// Preserve subresource suffix (e.g. /v1/responses/compact).
 		if suffix := responsesSubpathSuffix(rawRequestPath); suffix != "" {